WASHINGTON — Deep-seated racial misgivings could cost Barack Obama the White House if the election is close, according to an AP-Yahoo News poll that found one-third of white Democrats harbor negative views toward blacks — many calling them “lazy,” “violent” or responsible for their own troubles.

The poll, conducted with Stanford University, suggests that the percentage of voters who may turn away from Obama because of his race could easily be larger than the final difference between the candidates in 2004 — about 2.5 percentage points.

Certainly, Republican John McCain has his own obstacles: He’s an ally of an unpopular president and would be the nation’s oldest first-term president. But Obama faces this: 40 percent of all white Americans hold at least a partly negative view toward blacks, and that includes many Democrats and independents.

More than a third of all white Democrats and independents — voters Obama can’t win the White House without — agreed with at least one negative adjective about blacks, according to the survey, and they are significantly less likely to vote for Obama than those who don’t have such views.

Such numbers are a harsh dose of reality in a campaign for the history books. Obama, the first black candidate with a serious shot at the presidency, accepted the Democratic nomination on the 45th anniversary of Martin Luther King Jr.’s “I Have a Dream” speech, a seminal moment for a nation that enshrined slavery in its Constitution.

“There are a lot fewer bigots than there were 50 years ago, but that doesn’t mean there’s only a few bigots,” said Stanford political scientist Paul Sniderman who helped analyze the exhaustive survey.

The pollsters set out to determine why Obama is locked in a close race with McCain even as the political landscape seems to favor Democrats. President Bush’s unpopularity, the Iraq war and a national sense of economic hard times cut against GOP candidates, as does that fact that Democratic voters outnumber Republicans.

The findings suggest that Obama’s problem is close to home — among his fellow Democrats, particularly non-Hispanic white voters. Just seven in 10 people who call themselves Democrats support Obama, compared to the 85 percent of self-identified Republicans who back McCain.

The survey also focused on the racial attitudes of independent voters because they are likely to decide the election.

Lots of Republicans harbor prejudices, too, but the survey found they weren’t voting against Obama because of his race. Most Republicans wouldn’t vote for any Democrat for president — white, black or brown.

Not all whites are prejudiced. Indeed, more whites say good things about blacks than say bad things, the poll shows. And many whites who see blacks in a negative light are still willing or even eager to vote for Obama.

On the other side of the racial question, the Illinois Democrat is drawing almost unanimous support from blacks, the poll shows, though that probably wouldn’t be enough to counter the negative effect of some whites’ views.

Race is not the biggest factor driving Democrats and independents away from Obama. Doubts about his competency loom even larger, the poll indicates. More than a quarter of all Democrats expressed doubt that Obama can bring about the change they want, and they are likely to vote against him because of that.

Three in 10 of those Democrats who don’t trust Obama’s change-making credentials say they plan to vote for McCain.

Still, the effects of whites’ racial views are apparent in the polling.

Statistical models derived from the poll suggest that Obama’s support would be as much as 6 percentage points higher if there were no white racial prejudice.

But in an election without precedent, it’s hard to know if such models take into account all the possible factors at play.

The AP-Yahoo poll used the unique methodology of Knowledge Networks, a Menlo Park, Calif., firm that interviews people online after randomly selecting and screening them over telephone. Numerous studies have shown that people are more likely to report embarrassing behavior and unpopular opinions when answering questions on a computer rather than talking to a stranger.

Other techniques used in the poll included recording people’s responses to black or white faces flashed on a computer screen, asking participants to rate how well certain adjectives apply to blacks, measuring whether people believe blacks’ troubles are their own fault, and simply asking people how much they like or dislike blacks.

“We still don’t like black people,” said John Clouse, 57, reflecting the sentiments of his pals gathered at a coffee shop in Somerset, Ohio.

Given a choice of several positive and negative adjectives that might describe blacks, 20 percent of all whites said the word “violent” strongly applied. Among other words, 22 percent agreed with “boastful,” 29 percent “complaining,” 13 percent “lazy” and 11 percent “irresponsible.” When asked about positive adjectives, whites were more likely to stay on the fence than give a strongly positive assessment.

Among white Democrats, one-third cited a negative adjective and, of those, 58 percent said they planned to back Obama.

The poll sought to measure latent prejudices among whites by asking about factors contributing to the state of black America. One finding: More than a quarter of white Democrats agree that “if blacks would only try harder, they could be just as well off as whites.”

Those who agreed with that statement were much less likely to back Obama than those who didn’t.

Among white independents, racial stereotyping is not uncommon. For example, while about 20 percent of independent voters called blacks “intelligent” or “smart,” more than one third latched on the adjective “complaining” and 24 percent said blacks were “violent.”

Nearly four in 10 white independents agreed that blacks would be better off if they “try harder.”

The survey broke ground by incorporating images of black and white faces to measure implicit racial attitudes, or prejudices that are so deeply rooted that people may not realize they have them. That test suggested the incidence of racial prejudice is even higher, with more than half of whites revealing more negative feelings toward blacks than whites.

Researchers used mathematical modeling to sort out the relative impact of a huge swath of variables that might have an impact on people’s votes — including race, ideology, party identification, the hunger for change and the sentiments of Sen. Hillary Rodham Clinton’s backers.

Just 59 percent of her white Democratic supporters said they wanted Obama to be president. Nearly 17 percent of Clinton’s white backers plan to vote for McCain.

Among white Democrats, Clinton supporters were nearly twice as likely as Obama backers to say at least one negative adjective described blacks well, a finding that suggests many of her supporters in the primaries — particularly whites with high school education or less — were motivated in part by racial attitudes.

The survey of 2,227 adults was conducted Aug. 27 to Sept. 5. It has a margin of sampling error of plus or minus 2.1 percentage points.

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Associated Press writers Nancy Benac, Julie Carr Smyth, Philip Elliot, Julie Pace and Sonya Ross contributed to this report.