The Bradley Effect

With the presidential debates in the books and a commanding lead in the polls, Barack Obama appears to be coasting toward history. But a potential cakewalk makes for dull punditry, and politicos are abuzz over the last hurdle Obama must clear in his path to the presidency: a phenomenon known as the "Bradley effect."

The theory holds that voters have a tendency to withhold their leanings from pollsters when they plan to vote for a white candidate instead of a black one. In 1982, Tom Bradley—the African-American mayor of Los Angeles—ran for governor...