Bradley effect

The Bradley effect refers to the sometimes significant discrepancy between voter opinion polls and actual election results when candidates are of different races.

Origins
It is named for Tom Bradley, an African-American who lost the 1982 California governor's race despite being ahead in voter polls. Less commonly called the Wilder effect, it reflects a tendency on the part of white voters to tell pollsters that they are undecided, or likely to vote for a black candidate, to make a show of how enlightened and progressive they are. Then on election day, in the privacy of the voting booth, they think "the hell with that" and choose the white candidate.

Cause
There is indeed a tendency for people to lie through their back teeth fib a little try to make themselves look better tell massive, whopping great lies in surveys and opinion polls, particularly if a response would otherwise be frowned upon. This has to be taken into account by psychologists doing studies on sensitive subjects where, even though they're anonymous, people may outright lie, even as a form of denial. In the particular case of the Bradley effect, it is due to the risk of being perceived as a racist that people may claim support for a candidate of an ethnic minority.

Barack Obama
In recent years there was discussion over whether Barack Obama was experiencing the Bradley effect. In some cases during the Democratic primaries, the Bradley effect was reversed, while in others it may have been a factor. The difference between open and private polling may have also been a cause. However, some people still very publicly smear Obama because he's black.