Gallup, and its media sponsor,
USA Today, asked that very question and provided this: "To sum up the answer,
Democratic voters simply changed their minds.
"Second, the weekend poll that showed Obama leading Hillary Clinton by a substantial margin among Democrats was also the exact same poll that correctly and almost precisely predicted John McCain’s 5-point margin of victory over Mitt Romney in the GOP voting.
"If there had been a sampling or methodology problem with the poll, it logically should have affected both races. It didn’t. The discrepancy was between what Democratic voters told us in the poll and what Democratic voters actually did on Election Day. There was no discrepancy among Republican voters and what they did on Election Day.
"Which leads us to the second category of explanations for the disconnect between the pre-election poll results on the Democratic side and the actual voting. New Hampshire Democratic voters may have confounded us pollsters by actually changing their minds in the days and hours leading up to their vote. This is unusual. In most pre-election environments, voter statements of their vote intentions in the days before an election are good indicators of how they actually vote. But last minute changes do occur, which is why Gallup usually interviews the final Monday night before the national presidential election (but not typically, or this year specifically, in New Hampshire).
"There was a tremendous interest in this election among New Hampshire voters this year, as demonstrated by the record high turnout. This high level of interest suggests that voters may have been closely attuned to election events in the final days before the election, which in turn may them more likely to be affected by such events.
"The most obvious such event on the Democratic side was the extraordinary and very frequently aired video of Hillary Clinton’s personal, emotional, 'verge of tears' response to a female voter's question on Monday. At this juncture, it is impossible to determine the actual impact of this event on Democratic voting behavior, but it is not at all unreasonable to assume that it did cause some voters, including women voters, to change their minds about voting for Clinton.
"There was also the Saturday night Democratic debate, which allowed Democratic voters to contrast and compare Clinton and Obama side by side, and which may also have generated some changes of opinion among voters.
"Some have also argued that white voters in New Hampshire told pollsters they were going to vote for a black candidate (Obama) but when actually in the voting booth ended up not wanting to do so. This hypothesis has been evoked over the years in elections with black candidates, but has been difficult to prove. Discrepancies between pre-election polls and the actual vote for black candidates, in fact, are certainly not the norm. This year, as an example, the pre-election polls in Iowa were very accurate in relationship to Obama's actual vote in the caucuses."
