WASHINGTON (CNN) -- As both Sens. Barack Obama and Hillary Clinton race to make history, some political observers believe Obama might have a unique problem because he's African-American.
Sen. Barack Obama says he made inroads with working-class whites in the Pennsylvania primary.
They claim voters often say they'll go one way, then act quite differently at the polls when it comes to a candidate's race.
Obama's crossover appeal didn't seem to work so well in Pennsylvania, where he lost a significant portion of working-class whites and Catholic voters to rival Clinton.
One theory bandied about is that Obama suffered from what was once called the "Bradley" effect, later called the "Wilder" effect.
Tom Bradley in California and Douglas Wilder in Virginia are African-Americans who ran for statewide office in the 1980s.
Pre-election telephone polling showed them with more support than they actually got on election day.
But Keating Holland, CNN polling director, said that hasn't been seen with African-American politicians since then. Watch more on Obama's loss in Pennsylvania »
Even though Obama had been closer to Clinton in some pre-election Pennsylvania polls a couple of weeks before the April 22 primary, his debate performances and his referral to some state voters as "bitter" might have done more to hurt him than anything else. Watch more on exit polls from Pennsylvania »
"The primary polling that was held just on the eve of the election, the weekend before the election, tended to show Obama losing by 9 points, 10 points," Holland said.
"There is no indication in the polls that were taken immediately before the Pennsylvania primary that there was any sort of a 'Bradley' effect going on."
Democratic strategists say the notion of Obama having a problem with working-class whites is overstated; that the racial divide has dissipated.
But strategist Peter Fenn said, "When people get into the voting booth, they may have said to folks, 'I'll vote for an African-American', just like they would have said 'I'd vote for a woman' or 'I'd vote for a Hispanic.' And they don't do it. And we see a little bit of a drop-off still."
Strategists and pollsters caution not to read too much into Pennsylvania's results.
They point out Obama did very well among white voters in Iowa, Wisconsin and Virginia, where one pollster said if Obama is the nominee, he may well be able to challenge Republican dominance there. E-mail to a friend