When respondents were asked by the WSJ whom they voted for in the 2008 presidential elections, 41 percent said they voted for Obama, compared with 32 percent for McCain. Factor out the 18 percent who said they didn't vote, and you've got Obama beating McCain by 11 points, 50 percent to 39 percent.
The gap in the New York Times poll is even wider. In it, 48 percent of respondents said they voted for Obama, compared with 25 percent for McCain. Again, subtract the 19 percent who say they didn't vote, and you've got Obama winning by a nearly 2-to-1 margin, with 60 percent to McCain's 32 percent.
It is an old story. In Attitudes and Opinions (Routledge 2005), Oskamp and Schultz offer several examples of the social desirability bias. In a 1964 survey, 64 percent reported that they had voted for JFK four years earlier, even though he actually won only 49 percent of the vote.