Let us swing the door ajar and invite the elephant into the room. One big reason Barack Obama is locked in a tight race, rather than easily outdistancing his opponent, is because he is black.

That factor is rarely discussed in polite political conversation. People tend to dance around it, talking instead about Obama's perceived inexperience, or his youth, or his perceived airs, or his liberal voting record. And racist sentiment rarely shows up in the polls, because a lot of people don't want to share their baser instincts with the pollsters; they'll save that instead for the privacy of the voting booth.

But the incremental evidence - anecdotal and even statistical - has become impossible to ignore.

Union organizers in the key state of Michigan complain in the press that, as one puts it, "we're all struggling to some extent with the problem of white workers who will not vote for Obama because of his color." An aging mine electrician from Kentucky is quoted as saying, "I won't vote for a colored man. He'll put too many coloreds in jobs." An elderly woman in a New Jersey hair salon is overheard complaining about Barack and Michelle Obama the other day, about how blacks supposedly have larger bones than whites, and about how she's fleeing America if Obama wins.

Jimmy Carter, while attending the Democratic convention, cited race as a "subterranean issue," yet at times this year it has been bared for all to see. Case in point, Pennsylvania. On the day of the Democratic presidential primary, 12 percent of the white Democratic voters told the exit pollsters that race mattered in their choice of candidate; of those whites, 76 percent chose Hillary Rodham Clinton over Obama. The same pattern surfaced in other states, including the key autumn state of Ohio.

This is worth pondering a moment longer. If 12 percent of Democratic voters are willing to tell exit pollsters, eye to eye, that race was an important factor, to Obama's detriment, isn't it fair to assume that the real percentage (including those who kept their sentiments private) was actually higher? And what might this portend for the general election, when the white electorate will be broader, and hence significantly less liberal, than in Democratic contests?

Here's one hint. Last June, the Washington Post-ABC News poll devised a "racial sensitivity index," based on a series of nuanced questions that were designed to measure the varying levels of racial prejudice in the white electorate. The pollsters came up with three categories, ranging from most to least enlightened. The key finding: Whites in the least-enlightened category - roughly 30 percent of the white electorate - favored John McCain over Obama by a ratio of 2-1.

A few prominent Democrats did broach this sensitive topic at the Denver convention. Dee Dee Myers, the former Bill Clinton aide, shared her concerns at one political forum, and with good reason. She worked for Los Angeles Mayor Tom Bradley in the 1980s, when it appeared that Bradley was a cinch to win his U.S. Senate contest despite his race. The final round of polls showed him winning comfortably. He lost.

"I lived through that," Myers said. "We're whistling past the graveyard if we think that race was not a factor in the Democratic primaries. Today's young voters will get us past these attitudes," but it will take time. As for millions of older voters, "they talk about having 'culture' problems [with Obama], but to separate culture from race is impossible."

And Markos Moulitsas, who runs the liberal Daily Kos blog, said: "It's human nature. A lot of people want to cling to the comfortable world that they've always lived in. The Obamas don't look like what First Families have always looked like. This will be one of the factors in the fall, because a lot of people simply want to stick with what they've known in the past."

The race obstacle is not necessarily fatal, of course, because in the end it may be trumped by other factors - such as McCain's age, or nagging concerns about handing the nuclear football in an emergency to a "hockey mom" as GOP vice presidential candidate whose chief national security credential is the proximity of Alaska to Russia.

But clearly Obama needs to tread carefully, arguably by stressing lunch-pail economic issues and continuing to present himself as a "post-racial" candidate. He will need to dispel these white suspicions, if only because whites will continue to dominate the electorate - they constituted 77 percent of all voters in 2004 - even if he manages to inspire an historic black turnout. He has to bond somehow with blue-collar whites, yet he cannot show too much passion, because, as Democratic strategist Joe Trippi explained to me, "those whites don't like to see a black guy getting angry, it's a dangerous thing for an African American candidate to do."

I'm not suggesting that racism would be the sole explanation for an Obama loss. Nor am I seeking to insult those who object to Obama purely on the issues. But if Obama winds up losing after having posted a seemingly solid polling lead on election eve, we may well find ourselves pondering the words of Henry David Thoreau, who wrote in 1854 that "public opinion is a weak tyrant, compared with our own private opinion."

Contact Dick Polman at dpolman@phillynews.com. See his blog at http://go.philly.com/polman.