Brendan Nyhan already knocked this down in up to 140 characters, but I’ll take a shot too...

John Heilemann spins a convoluted scenario in which Sarah Palin wins the presidency in 2012 as a consequence of a third-party run from NYC Mayor Michael Bloomberg. Now, technically I suppose this doesn’t violate the Iron Law of Politics that New York City Mayor is a dead-end job; NYC Mayors do occasionally run for other offices and lose. Sometimes, they run for president. Then they get crushed:

  • John Lindsay sort of tried for the Republican nomination in 1968, but it didn’t go anywhere.

  • By 1972, Lindsay was a Democrat, and over the course of the primaries wound up getting about half the votes of Shirley Chisolm (sorry, Betty Draper. It’s not gonna happen).

  • And Rudy Giuliani, you know about.

To be fair, Heilemann doesn’t have Bloomberg -- who, let’s remember, has neither constituency nor, at this point, any issues to run on -- winning. And it’s hardly insane to imagine that if Barack Obama is unpopular in 2012 , the right crazy billionaire could wind up doing about as well as Ross Perot did in 1992. On the other hand, Perot caught a few breaks (mainly, getting included in the presidential debates thanks to a blunder by George H.W. Bush’s campaign), and while surely he was crazier than Bloomberg would be, he also was more culturally compatible with swing voters than the short Jewish guy New Yorker.  (Being crazy may have been an advantage, too, in that Bloomberg might not be nearly as willing to spend hundreds of millions on a pipe dream).

But the real point here is that the third party fantasy is irrelevant. If there’s a double-dip recession and unemployment is over 10%, then odds are good that any GOP nominee would win.

Yes, Palin is very unpopular right now, no question about it. That hurts her chances of winning the nomination. But if she does manage to do that, well, we’re not talking about today’s Palin any more. Republicans would rally around her; newsmagazines and network news poobahs would do features on how she’s grown since ‘08; I don’t need to tell you how enthusiastic the folks at Fox News and conservative talk show hosts will be. She will have won GOP presidential debates (doesn’t matter how she performs; since we’re assuming she’s nominated, that means she won the primaries that followed the debates, which means she’ll be declared the winner).

Now, in a world in which unemployment is dropping, and Barack Obama has approval ratings of 55% or higher, then Palin would enter the general election behind and stay behind, and reporters (other than the partisan press) would rapidly forget her alleged growth and focus on how extremist and gaffe-tastic her campaign continued to be. In that world, Obama is re-elected in a landslide.

If, however, Obama is under 40% approval, then Palin enters the campaign leading, and her campaign is thrilling and exciting -- a phenomenon! Swing voters who already gave up on the president would either find themselves reconsidering the Sage of Wasilla (who, remember, would be getting great press), or would reluctantly settle for her.

Would a third-party candidate change any of that? In those cases, not at all. 

Could it possibly make a difference? Never say never...if a third party candidate runs to the left of a marginally popular Barack Obama, it’s certainly possible to imagine such a candidate taking 10% of the vote, all from the Democratic nominee. Unlikely -- a lot of things have to happen to maximize the vote of the splitter, and the two-party race has to be close -- but not impossible. Of course, given the record we’ve seen in 2010, right now it seems more likely that we’ll see that on the right, not the left, but the opposite isn’t impossible.

However, a third party coming from the center, more or less, won’t do that at all; to the extent it “succeeds” by reaching double digits, odds are that those voters will come more or less equally from both parties. After all, that’s sort of the point of such a candidacy, to appeal to those who find their party drifting too far to the extremes. To the extent that one of the candidates is particularly weak, a center-based third party candidate might draw more from that side, but that’s just a sign of a candidate’s weakness -- which, without the third party candidate, the other side would have exploited.

Indeed, a candidate who is flexible enough to have choices about where to position his campaign ideologically is going to search for his oppponents’ weaknesses. So if Obama is weak, Bloomberg might well run center-left -- but that just implies that without the third party candidate, the Republican nominee would have the advantage. If that GOP nominee is the weak one, Bloomberg would position center-right, and poach (to the extent he could) her voters. All of which makes for a lot of fun for reporters, but doesn’t change the outcome. 

In short, it’s very unlikely that a third party candidacy would throw an election to a candidate who would otherwise lose; it’s even more unlikely that center-based third party candidate would do so; and as much as a big checkbook impresses reporters, there’s no real reason to think that Michael Bloomberg is particularly well positioned to be a particularly effective center-based third party candidate.