Far from the storied hollows of Appalachia, and well before the Rev. Jeremiah Wright lit up the cable news channels or the “elitist” label was fixed to Barack Obama’s lapel right where his flag pin ought to have been, a group of older white voters drew a firm line on his candidacy. On Feb. 5, Mr. Obama prevailed by just 10,000 votes in Missouri, a general election swing state. His victory margin was cramped by the fact that he lost white voters over age 60 by nearly 40 percentage points, 67 percent to 28 percent. And it had nothing to do with bowling.

It’s possible that older white people just don’t know where Barack Obama is coming from. Then again, perhaps the problem is they do. One useful gauge of racial tolerance over the years has been the percentage of Americans who approve of interracial marriage. In 1961, when Mr. Obama’s African father and white mother were married, they joined an exceedingly small and extremely unpopular minority. According to the 1960 census, of more than 40 million married couples living in the United States that year, a mere 51,000, or 0.1 percent, consisted of a black and a white. A Gallup poll from 1958, just three years before Mr. Obama was born, found that only 4 percent of white people approved of such marriages, one point greater than the poll’s margin of error.

Both racial attitudes and the frequency of interracial marriage have changed significantly since then. But approval is far from universal. A 2007 Gallup poll found that among whites over age 50, less than two thirds, or 64 percent, approve of marriage between a black person and a white person. Subtract a few points for those who were reluctant to appear intolerant on the survey, and you wind up with roughly 4 in 10 white people over age 50 refusing to support interracial marriage. It hardly seems far-fetched that voters who oppose black-white unions in the first place might have some difficulty supporting the product of such a union as their president.

Mr. Obama has positioned himself as a man largely freed from 400 years of racial perversity. When he speaks of racism, it’s usually as something experienced by others who came before him. The implication is that Mr. Obama himself was born too late to suffer more than random, glancing blows. Racism, it seems, is mostly something he has read about in books.

For Mr. Obama, this is a shrewd political stance. But can it really be an accurate description of his 46 years? To accept that, we must believe that the young Obama, a political prodigy in formation, remained insensitive to the opprobrium society heaped upon his own family, whose multiracial composition was opposed by a majority of Americans — and despised by a vocal white supremacist element —through much of his life. It wasn’t until 1997 that a Gallup poll registered white majority support for black-white unions, by which time Mr. Obama was 35. Even in the Gallup survey taken last year, only three-quarters of Americans said they approved of mixed marriages. In a nation of 300 million, that leaves quite a large helping of racial ambivalence and obstinacy still on the table.

Mr. Obama’s self-described “improbable journey” to the Democratic nomination has confounded many assumptions about race and the American electorate. Many white primary voters, especially the young, clearly considered Mr. Obama’s race an asset to cheer rather than an obstacle to overcome. And Mr. Obama now has an opportunity to draw support from ideological and religious conservatives who are racial liberals, a seemingly expanding minority discomfited by the American, and more recent Republican, histories on race.

To realize his ambition, however, Mr. Obama must not only soothe the racial rear guard, whose anxieties can so easily leach into the electoral mainstream, he must pretend their prejudice is the faint residue of a bygone era. Every candidate for president offers plans for taxes and spending. But who other than Mr. Obama has offered to wash away so many sins with one stroke? Is it any wonder he is the subject of Christian iconography — both sincere and mocking?

As a young man, Mr. Obama recognized that he would have difficulty functioning as half white, half black in a racially polarized society. He made a choice, one predetermined to a significant degree by the color of his skin. If the wounds from that painful process and from his racially complicated youth seem to have magically healed, it’s not because he has rolled away the heavy stone of prejudice and ushered in a post-racial morning in America. It’s because Mr. Obama views public denial of his own life experience as a political imperative. To succeed, he has trained his formidable will not just on changing the future, but the past.