So if the conditions are so ripe for an Obama victory, why is the race so close?
Because millions of white Americans, especially those who are forty-five and older, may not be able to bring themselves to vote for the black guy. It’s that simple.
Recent research, however, is casting doubt on the "Wilder Effect" as applied to the Democratic primaries:
The 2008 Democratic presidential primaries renewed speculation about the Wilder effect, so as a final test, we applied the same decision rules as above to collect up to three polls for each of the 33 U.S. states that held contested Democratic primaries. Doing so yields 87 observations of polled and actual support for Senators Barack Obama and Hillary Clinton. The mean polling-performance gap was 1.4 percentage points for Senator Clinton, and the reverse for Senator Obama. This estimated mean is not at all sensitive to particular polls or states: if we remove the observations for ve randomly chosen states at a time, we still observe that Senator Obama’s election-day performance was better than his polling on average in every one of the 10,000 simulations. This is yet more evidence that the Wilder effect, strong in the early 1990s, is strong no longer.Check out the original research paper, "No More Wilder Effect, Never a Whitman Effect: When and Why Polls Mislead about Black and Female Candidates."
See also, "Do Voters Tell Pollsters the Truth in Racial Surveys?"