John Weaver hunches his angular frame over a Styrofoam cup of coffee in the basement cafeteria of the United States Senate and tries to explain what might seem--to an outsider--his peculiar political loyalties. Once a loyal Republican strategist who directed the presidential aspirations of uber-conservative Phil Gramm and helped plot John McCain's maverick primary run in 2000, he has since re-registered as a Democrat and severed consulting ties to all Republicans except McCain, for whom he still serves as chief strategist. "I only work for Democrats now," he tells me. Noticing that he has overlooked the party affiliation of his most prominent advisee, I helpfully add: "And John McCain." Weaver shrugs his shoulders and grins, "Oh, right."

It's easy to forget that the Arizona senator is not, in fact, a Democrat. In the past year he has stood against his party on so many prominent and contentious issues that his concurrences with GOP dogma have become more of an exception than a rule. In the conservative media, he has become a figure of vilification on par with Tom Daschle. Last fall, when his name came up in a meeting of House Republicans, he was booed. And it is no exaggeration to say that he has co-sponsored virtually the entire domestic agenda of the Democratic Party. One prominent Democrat enthuses, "He's the leader of the loyal opposition." Typically that role falls to a leader of the opposition party. But the most popular and effective champion of the Democratic Party's values isn't Tom Daschle. It's John McCain. And at a moment when the party is casting about for a leader to define it against a popular president, and McCain is casting about for a home after his virtual expulsion from the GOP, there is an obvious solution to both dilemmas: John McCain ought to become a Democrat--and a presumptive front-runner for the party's presidential nomination in 2004.

If there's been relatively little discussion of McCain switching parties, it's largely because few have completely assimilated the extent of his political transformation. One reason is that McCain and his staff systematically pooh-pooh it. Conservatives attack McCain on ideological grounds, so his supporters respond, understandably, by downplaying the ideological nature of his apostasy--insisting, for instance, that he has merely gained a greater appreciation for bipartisanship. "I am fundamentally, philosophically, the same person that I have been," McCain told National Journal last summer. A second reason is that political reporters prefer personal or characterological explanations over ideological ones, and in McCain they have a perfect one: He and his staff still bear the scars of their bitter 2000 primary campaign against George W. Bush. Naturally, reporters interpret McCain's opposition to Bush's agenda--or Bush's opposition to McCain's--as merely a grudge match between the two former rivals.

In truth, though, McCain's objection to Bush's agenda stems from a genuine difference of belief about the nature of the public good. As I noted in these pages two years ago ("This Man Is Not a Republican," January 31, 2000), McCain's ideological evolution began fitfully and, to some extent, unconsciously. McCain ran for Barry Goldwater's seat in the Senate in 1986, promising voters they'd never know the difference. Yet he gradually developed a maverick streak and, by the late Clinton years, was crusading on behalf of campaign finance reform and restrictions on the tobacco industry. Still, McCain remained a Republican in good standing until his presidential bid, which forced him to formulate positions on domestic issues that, as he explained to me at the time, he had never before spent much time pondering. When he did, he found that his instincts to advance the broader interest over individual entitlement left him in favor of the sort of moderate fiscal conservatism advanced by President Bill Clinton and opposed to the traditional GOP nostrum of upper-bracket tax cuts. It was his tax cut heresy that ultimately rendered McCain a pariah among the party elite. The conservative interests that form the GOP base struck at him with a vengeance, most notably in South Carolina; and the ferocity further alienated McCain and his loyalists. Pretty soon McCain was veering off in directions nobody could have foreseen even a few months before, openly pointing out that Bush's tax cut favored the rich and attacking influential religious conservatives like Pat Robertson and Jerry Falwell as "forces of evil." As Marshall Wittmann, who advised McCain during the primary, puts it, "Ideologically, we all changed."

The degree to which McCain has abandoned contemporary conservatism is reflected in the legislative program he has championed since Bush took office. Most notably, of course, he shepherded campaign finance reform--an effort that put him in close cooperation with Democrats in Congress. McCain also collaborated with liberal Democrats John Edwards and Ted Kennedy on a patients' bill of rights; with Charles Schumer on more widespread sale of generic prescription drugs; with Ernest Hollings to put federal employees in charge of airport security--all of which set him against fierce business lobbying. And he teamed up with Evan Bayh to promote AmeriCorps, an effort Bush later co-opted with his own smaller AmeriCorps boost.

But perhaps most amazing has been McCain's willingness to take stands even many Democrats are afraid of. He voted against Bush's tax cut, the centerpiece of the new president's agenda. Along with John Kerry, he sponsored legislation to raise automobile emissions standards, and he paired with Joe Lieberman to try to force Bush to reduce greenhouse gases in compliance with the Kyoto accord. Also with Lieberman, McCain has proposed forcing people who buy firearms at gun shows to undergo background checks--closing the "gun-show loophole"--even as most Democrats shy away from any form of gun control. He has infuriated the gambling industry by proposing to ban wagering on college sports. And along with Carl Levin, he has co-sponsored a bill to force companies that deduct executive stock options from their taxes to disclose the cost on their financial statements--another effort few Democrats have been willing to join.

One conclusion to draw from all this is simply that McCain has guts. But there's more to it than that. McCain's domestic agenda increasingly consists of bold reforms that expand the scope of the federal government. During the campaign, McCain paid lip service to anti-government bromides while supporting government intervention in specific instances. In the last year, though, his ideology has grown coherently progressive. "We have had regulatory agencies always to curb the abuses or potential abuses of the capitalist system," he said earlier this year on CBS's "Face the Nation." "This is not a totally laissez-faire country." McCain, in other words, now believes in progressive government to counteract the excesses of the market and recognizes that the mere fact that business interests complain about such intervention does not by itself make it wrong. There is a term for people who think like this: Democrats.

So why does McCain remain in the GOP? Aside from sentimental attachment, McCain idolizes Teddy Roosevelt and sees his mission as reshaping the GOP in his hero's image: progressive; crusading; independent from (and frequently at odds with) business; willing to use federal power for the greater good; and representing all economic classes, not just the rich. It's a noble aim. Alas, it isn't working. Under Bush, the GOP has grown more wedded to business interests than at any time since the 1920s. The White House and K Street coordinate policy so tightly that their agendas are frequently impossible to separate. And on high-profile issues, McCain's legislative coalitions consist entirely, or almost entirely, of Democrats.

McCain's invocation of Roosevelt, of course, has a second purpose: It suggests that he, like T.R., might bolt the Republican Party and launch an independent presidential bid. It doesn't take a great deal of imagination to read this possibility into some of McCain's comments. Last summer, for example, he told Newsweek, "There is a growing vacuum out there. Unless the two parties move to fill that vacuum, you will see the rise of independent candidates." And while McCain has since clammed up about running for president, the idea of an independent bid remains alive among his admirers. Wittmann, who works at the Hudson Institute but remains close to McCain, publishes a daily Internet commentary under the title "The Bull Moose," in which he teases readers with periodic hints of a McCain independent run.

A McCain independent candidacy, however, would stand little chance of winning. Since his primary run, polls show McCain's popularity has grown among Democrats while collapsing among Republicans. As a result, a McCain third-party run would probably guarantee Bush's reelection. The latest poll on this, taken in June, showed a match between Bush and a generic Democrat splitting voters evenly at 44 percent each. If McCain were to enter the race, though, almost all his support would come at the Democratic nominee's expense--the Democrat would drop to 29 percent, while Bush would fall to just 42 percent, with McCain trailing both at 22 percent. While one shouldn't read too much into these numbers--it is, after all, a poll of a hypothetical matchup more than three years in advance of the actual election--it is hard to imagine an independent McCain run having any effect other than to split the center-left vote. The basis of his candidacy, after all, would be a populist critique of Bush for putting special interests ahead of the broader good--almost certainly the same theme a Democrat would take up. If Bush could merely hold onto his base, with whom he's extremely popular, he'd win. The essential problem with an independent McCain candidacy is that his political profile, like his profile as a legislator, has become that of a hawkish Democrat. Which raises an obvious, if rarely discussed, possibility: Why shouldn't McCain run for president as a Democrat?

It's strange that amidst all the political gossip about a McCain third-party candidacy, Washington has barely considered the far more logical possibility of a McCain second-party candidacy. Last summer one senator informed Newsweek that McCain himself has discussed the idea. "'I know he's looked at our field and said to himself, 'I could take these guys,'" the (presumably Democratic) senator explained. Yet the capital's political rumor industry, which usually requires only the flimsiest scrap of data to launch a speculative frenzy, completely ignored this tidbit. The conventional wisdom seems to be that Democrats wouldn't accept McCain. Elizabeth Drew, whose recent book Citizen McCain reflects the McCainiac worldview, writes, "McCain, with his conservative record on issues such as abortion, would never be welcome in the Democratic Party." But while McCain's historical voting record may be conservative, his recent record in Congress is anything but. In off-the-record conversations, Republicans argued that Democrats would never accept McCain, while Democrats--even very liberal ones--said just the opposite. The assumption that McCain is at ideological loggerheads with the Democratic Party either misunderstands McCain, or the Democratic Party, or both.

Some of McCain's supporters, for instance, argue that his interventionist views on foreign policy would render him unacceptable to Democratic primary voters. But while some liberal elites may harbor misgivings about McCain's muscular internationalism, there is no mass constituency for dovish policies these days, even among Democrats. And it's easy to exaggerate the foreign policy cleavage between the two parties. In recent years the boundaries of the foreign policy debate have grown extremely fluid, with Democrats and Republicans switching back and forth depending in large part upon whose party held the White House. It wasn't long ago, after all, that a Democratic president was urging military intervention (in the Balkans), while Republicans in Congress were appealing to isolationism. The political class in both parties remains oriented around domestic policy. It's far easier to join a party with whom you already agree on the role of government at home and change its foreign policy than to do the reverse.

And the most prominent feature of Democratic foreign policy since September 11 is that there isn't much of one. Yes, a couple Democrats--mostly old cranks like Robert Byrd and Hollings--have worried about an open-ended conflict; but others--such as Lieberman--have staked out terrain to Bush's right. The general mood among Democrats in Washington is to lay low on foreign affairs and to confront Bush in the domestic arena. Not only does this mean that McCain's hawkishness would pose little barrier to his nomination; it also presents him with an opportunity to determine what kind of Democratic foreign policy will emerge in the wake of the war on terror. And here McCain has a chance to shape the future of American politics--which, like all things historical, can be highly contingent. After all, if Franklin Roosevelt hadn't replaced Henry Wallace with Harry Truman as his vice president, the Democratic Party would not have built its policy of containment in the two decades after World War II. In the post-post Vietnam era now beginning, McCain could redefine the Democratic Party once again as the champion of Wilsonian interventionism.

The more serious objection is McCain's positions on social policy, especially abortion. Political analysts in both parties told me his pro-life views would cripple him in a Democratic primary. Yet this ought not be a disqualifying factor. Roe v. Wade is settled precedent. Keeping it from being overturned doesn't require liberal, pro-choice Supreme Court justices. It merely requires moderates who respect established precedent. Clinton did not nominate judicial activists; he nominated moderates who voted very much like Richard Nixon's and Gerald Ford's appointees. So long as McCain avoided appointing aggressive conservatives, there's no reason to think his appointments would have any practical effect.

Moreover, it has gotten hard to discern to what degree McCain is actually anti-abortion at all. At one point during his primary run, he told a reporter that "certainly in the short term or even the long term I would not support the repeal of Roe v. Wade." Another time, when asked what he would do if his daughter sought an abortion, McCain replied that he'd leave the final decision to her. In both instances, he restated his anti-abortion position after the ensuing uproar, but polls showed that voters believed he was pro-choice. In the last year McCain reversed himself and came out in favor of stem-cell research. So while it's hard to figure out where he stands, the best guess is that he remains personally against abortion but neutral, or even opposed to, making it illegal. Liberals might not find this position completely satisfying, but they ought to be able to live with it. Mario Cuomo took essentially this view of abortion, after all, and it did not prevent him from becoming a beloved figure among party activists.

Besides, McCain wouldn't need to be the favorite candidate of the National Abortion and Reproductive Rights Action League and People For the American Way. If McCain were to run as a Democrat, he would, as four years before, go over the heads of constituency groups and appeal directly to the grass roots. McCain has great appeal among both rank-and-file labor and the reform-minded upper-middle-class voters who flocked to the good-government messages of candidates like Bill Bradley and Paul Tsongas.

As in 2000, he would draw independent voters who, in the absence of a contested GOP primary, might support him in even higher numbers than last time. (What else would moderate Republicans do on primary day?) Even if the Democratic establishment rejected McCain altogether, as the GOP establishment did last time around, McCain could still win. Why? In 2000 anti-McCain Republicans rallied around a single alternative: Bush. In 2004 anti-McCain Democrats, lacking a single unifying choice, would split among several alternatives.

And there's another reason for even hard-core Democrats to swallow their ideological qualms about McCain: After September 11 he may be the only candidate who could beat Bush. It's not just that McCain retains wild popularity among the independents who decide elections. September 11 has transformed the political landscape in two critical ways. The first is widely understood: It erased doubts about Bush's legitimacy and competence. The second is less understood but even more important: It made foreign policy once again a crucial political criterion. During the last two decades of the cold war, Democrats won the White House only once, in large part because voters harbored doubts about their toughness on foreign affairs. Every Democrat had to clear this hurdle, and most--picture Michael Dukakis in the tank--failed to do so.

The natural Republican advantage on "toughness" remains. Consider the last election. Democrats nominated a sitting vice president who had enlisted in the military during wartime and earned a reputation as a hawk during the 1980s. Republicans nominated an inexperienced governor who avoided serving in Vietnam by taking refuge in the National Guard and who was known for flubbing the names of foreign countries and leaders. Al Gore's budget allocated more for defense than did Bush's. And polls showed the public still trusted the Republican more to handle defense. Yes, Demo crats carried the popular vote, as they have in all three elections since the end of the cold war. But that's exactly the point: The fall of the Berlin Wall reduced foreign policy's political importance, allowing Democrats to win on the basis of their more popular domestic positions. Since 9/11 the country has reverted to a cold war political environment, and popular domestic positions alone won't overcome the public's doubts about Democratic toughness. In different ways, Gore, Lieberman, and Kerry each has credentials that would give him a shot at overcoming this burden. But none has as good a shot as McCain--a former POW with an awe-inspiring personal story and a history of aggressively backing American military efforts.

When he ran for president two years ago, McCain liked to say that he patterned his views not only after Theodore Roosevelt but also--perhaps in part to protect himself ideologically--Ronald Reagan. His admirers still compare him to Roosevelt, but they rarely refer to Reagan anymore. The comparison they invoke--I have heard it a half-dozen times--is Henry "Scoop" Jackson, the prominent cold war Democratic senator. Both Roosevelt and Jackson offer cautionary examples for McCain.

Jackson's enthusiasts today come mainly from the ranks of conservatives who admire his hawkish commitment to using American power to promote democratic values across the globe. But Jackson believed just as deeply in using the government to promote a decent society on our own shores. On domestic policy he was a classic New Deal liberal, and, while McCain more closely tracks the thinking of New Democrats, he and Jackson share the same confidence in progressive, reformist government.

Yet Jackson's ultimate legacy is failure. He ran for president and lost in the primaries in 1972 and 1976, at a time when the Democratic Party's foreign policy remained in the grip of McGovernism. While he remained a forceful presence in Congress, he is remembered for representing what his party was not, rather than what it was. Roosevelt, too, provides a problematic model. He sought the presidency as an independent and lost as well, leaving the GOP dominated by its money wing. The lessons are clear: You change history not from Congress but from the White House, and not as an independent but as the leader of a major party in which your ideas can be institutionalized and carried on by others. Only a handful of politicians per generation capture the public's imagination and channel it toward moral and rational ends. McCain has the opportunity to do this. He can leave his imprint on history, but history will not come to him.