Bradley Effect Definition

A theory concerning observed discrepancies between opinion polls and election outcomes, according to which some white voters who intend to vote for a white candidate may tell pollsters that they are undecided or likely to vote for a non-white candidate, for fear of being accused of racism .

Wiktionary

Origin of Bradley Effect

Named after Los Angeles Mayor Tom Bradley, an African-American who lost the 1982 California governor's race despite being ahead in voter polls going into the elections.

From Wiktionary