The Bradley Effect

From Wikipedia:

The term Bradley effect or Wilder effect refers to a phenomenon which has led to inaccurate voter opinion polls in some American political campaigns between a white candidate and a non-white candidate.[1][2][3] Specifically, there have been instances in which statistically significant numbers of white voters tell pollsters in advance of an election that they are either genuinely undecided, or likely to vote for the non-white candidate, but those voters exhibit a different behavior when actually casting their ballots. White voters who said that they were undecided break in statistically large numbers toward the white candidate, and many of the white voters who said that they were likely to vote for the black candidate ultimately cast their ballot for the white candidate. This reluctance to give accurate polling answers has sometimes extended to post-election exit polls as well.

Researchers who have studied the issue theorize that some white voters give inaccurate responses to polling questions because of a fear that they might appear to others to be racially prejudiced. Some research has suggested that the race of the pollster conducting the interview may factor into that concern. At least one prominent researcher has suggested that with regard to pre-election polls, the discrepancy can be traced in part by the polls’ failure to account for general conservative political leanings among late-deciding voters.