Over at Real Clear Politics, Jay Cost tries to sort out whether Hillary or Obama is the better closer. His answer? It depends what you mean by "closer." Cost goes back and examines the "when did you make up your mind" question exit polls ask and finds that Hillary tends to out-perform her final, overall vote-share among people who made their decision more than a month out from Election Day and on Election Day, while Obama does better with those who decided in the last month, last week, and last three days.
Cost's theory for why day-of-election deciders break disproportionately to Hillary:
They probably either have paid so little attention to the campaign that it has had no effect on their thinking, or they have been paying attention but remain genuinely ambivalent. Either way, Clinton does better among them because she is so well known; both go to the candidate they know more about. My sense, then, is that Clinton does not so much "close well" - a phrase which implies that the shift is a consequence of her own efforts - as she benefits from having been a national figure for nearly two decades.
Maybe. My own theory has to do with the unique psychological dynamic I've talked about before, wherein voters don't want Hillary to win the nomination, but they don't want her to lose it either. Suppose this describes a lot of voters who are undecided heading into the last week or two of the campaign. Many of these voters migrate to Obama when he looks like the exciting, momentum-gaining underdog. Then, by the time Election Day rolls around, the people still undecided--or the people who once favored Obama but changed their minds--probably realized Hillary was about to lose and rallied around her.
P.S. We may get an interesting test of this theory on March 4. Hillary appears to have raised $35-$36 million in February, something the campaign announced today. In response, Obama spokesman Bill Burton told Ben Smith that the campaign has raised "considerably more." I suspect that means something north of $45 million (maybe 50), and that the Obama campaign will wait until Sunday or even Monday morning before announcing it in order to create a final burst of momentum. If I'm right about people not wanting Hillary to lose, that move could backfire, sending day-of deciders disproportionately to Hillary. (Of course, Obama could still win even if he does worse among late-deciders than he does among everyone else. That's happened in a lot of the states he's already won.)
P.P.S. Another data point in support of my theory: In the Iowa caucuses, at which point no one was concerned about Hillary getting bounced from the race, Hillary "closed" the worst of the three major candidates, according to Cost's definition (i.e., of ratio of day-of deciders to overall vote share). That suggests Hillary's strong closes have something to do with the fear she'll have to quit the race if she loses.
P.P.P.S. One other possible explanation for why Hillary seems to over-perform among day-of deciders: mean reversion. Obama has won a lot of these primaries with 60 percent or more of the vote. It's really hard to sustain that kind of margin for a long period of time--elections are dynamic phenomena. It wouldn't shock me if part of the day-of pattern we observe is just a reflection of that reality.
(In general, I think winners of any sort of contest tend to win by less than their largest leads, and losers tend to lose by less than their largest deficits. As Obama has won a lot more contests than Hillary, and as he typically builds his leads within a couple weeks of Election Day, you'd expect Election Day itself to be worse for him and better for her.)
Update: It looks like the CW is that Obama will raise around $50 million in January February [thanks to commenter thetraytiger for catching that]. That makes me think it'll be closer to 55 or 60 million, otherwise the Obama people wouldn't have let expectations get so high...
Also, there's one other thing that really hurts Obama if my theory is right: The increasing back and forth between him and McCain, which makes it seem like he's looking past Hillary. Obviously, a good chunk of this is beyond his control, since it's McCain who's initiating the exchanges. Still, the atmospherics clearly concern the Obama campaign, which is why you saw Obama himself invoking New Hampshire today.
--Noam Scheiber