Some years ago, C-SPAN's Brian Lamb played a dirty trick on a journalist friend of mine. My friend was appearing on C-SPAN's "Washington Journal" to discuss recent events. If I remember right, my friend was expanding on the future of East Timor when Lamb abruptly shifted the conversation. It was, Lamb announced, "vice-presidents week," at C-SPAN: surely one of the network's most successful ratings stunts--and, more importantly, an opportunity for Lamb to deploy his notorious deadpan humor.

"Tell us which vice president of the United States this is," Lamb commanded. A look of concern flashed across my friend's face as a sad-faced man with a drooping mustache appeared on the screen. My friend confessed his ignorance. "It's Garrett Hobart, McKinley's first vice president."

My friend struggled to return to East Timor. No dice. Later, Lamb cued a sepia photo of an elderly gentleman with bushy eyebrows tugging on an enormous cigar. "See if you recognize this fellow." My friend drew another blank. "That's John Nance Garner, former speaker of the House."

And so it went. In all the handicapper's excitement of the vice-presidential guessing game, it's easy to lose sight of the hard truth of American politics: It's rare that a vice-presidential choice makes any difference at all to the outcome of a race.

Presidential nominees have used vice-presidential choices to accomplish one or more of three main goals: unite their party, extend their demographic appeal, and balance some perceived personal deficiency. Thus Bob Dole chose Jack Kemp in 1996 to assuage the GOP right wing (ironically, the same reason that Gerald Ford had chosen Dole back in 1976). On the Democratic side, Jimmy Carter picked Walter Mondale in 1976 to reassure party liberals. The elder Bush opted for Dan Quayle in 1988 in hopes of wooing younger voters, much as Walter Mondale had chosen Geraldine Ferraro in an effort to mobilize women and George McGovern had hoped that Sargent Shriver would stanch his losses among Catholics. John Kerry chose John Edwards in 2004 to extend his appeal into the South and to balance his image as a scion of privilege.

None of these gambits worked. Edwards not only failed to deliver his own state to Kerry, but even his own county. Ferraro did not deliver women, Quayle did not deliver youth, and Catholics defected to Nixon in 1972.

Even the seeming exceptions to the rule only confirm it. In 1952, Adlai Stevenson chose John Sparkman of Alabama as his running mate. Reinforced by Sparkman's (tepid) segregationist voting record, the liberal Stevenson carried the Deep South. Four years later, Stevenson ran alongside the anti- segregationist Estes Kefauver--and once again carried the Deep South.

There is probably only one example in recent political history of a vice-presidential choice doing the chooser any real good: John F. Kennedy's selection of Lyndon Johnson in 1960, which greatly helped to return Texas to the Democratic column after the state had strayed in 1952 and 1956.

Yet, if vice-presidential nominations do little good for the candidate at the top of the ticket, they do great good for the number two, especially for Republican number twos. Since World War II, ten men have received the Republican nomination for vice president. Three of those men continued on to win the presidential nomination for themselves, and two actually became president. Meanwhile, a fourth nominee, Thomas Dewey's running mate Earl Warren, rose to arguably even greater power as chief justice of the U.S. Supreme Court. (Among Democrats, four of 13 veep nominees went on to head the ticket. Only one, Johnson, won--and he was already president when he ran.)

So as the nominee of my party, Senator John McCain, shortens his list, he should also lower his expectations. There is no running mate from the conservative wing of the party who can calm the internal anger that will be provoked if McCain reverts to his pre-primary support for so-called comprehensive immigration reform. There is no dark horse from the business community who can excite working-class voters in the absence of an exciting health care plan. This is one rare time in politics where policy in fact does count more than personnel.

Which means, Senator, you should forget about wooing evangelicals with a Mike Huckabee or women with a Carly Fiorina. Don't bother attempting to add financial expertise with a Michael Bloomberg or reaching out to party conservatives with a Mitt Romney. Don't delude yourself that a Condoleezza Rice can somehow act as a "gamechanger."

Instead, keep in mind that, when you choose, you may well be choosing a future leader of the party--a leader whose impact could well equal or exceed your own. Ronald Reagan's selection of George H.W. Bush as his running mate in 1980 resonates to this day, and Richard Nixon left a more enduring mark on Republicanism than Dwight Eisenhower ever did. You never know.

If you use the second slot to make peace with an internal opponent, your moment of comity could end by overturning everything you believe in. Had John Hinckley's bullet been aimed a little more accurately, the Reagan revolution would have lasted only 69 days.

Nor should you recruit a number two to act as your attack dog. Nixon later rationalized his Spiro Agnew choice with tough talk about how Agnew could say things about political opponents that would be unpresidential coming from Nixon. But the things Agnew said were (correctly) perceived as coming from Nixon. Leave the attacks to the 527s. You can plausibly disavow them if need be. You can never disavow your running mate without looking worse than negligent yourself.

I have my own personal nomination for vice president for McCain. It's Rudy Giuliani, precisely because he shares the vision of a practical, reforming, war-winning Republican Party that inspires John McCain, plus the stronger-than-usual grounds for hoping that he might be the rare candidate who can make a difference in an essential state--in this case, New Jersey.

But, my personal preferences aside, I hope, Senator, that you will make your choice with this consideration in mind: This choice may prove to be your most important legacy to your party and your country. Your nominee probably won't help your candidacy--but he or she may secure your vision or else destroy your reputation. Sports talk about the veepstakes is good, entertaining political fun. But a party in as much trouble as the GOP in 2008 has little time for fun.

David Frum, a resident fellow at the American Enterprise Institute and a speechwriter to President George W. Bush from 2001–2002, is the author most recently of Comeback: Conservatism that Can Win Again.

Click for the previous article in TNR’s Veeptacular. And for a full index of our 2008 v.p. coverage, click here 

By David Frum