So now it's all but official: The Democratic presidential nominee this year will be Sen. Barack Obama of Illinois. And facing him, it has been clear for several months, will be the Arizona Republican Sen. John McCain.

On reflection, it seems clear that Obama was the all-but-inevitable Democratic choice. The Clintons are a formidable pair, and Hillary put up a memorable battle. She also had what, in this day and age, would ordinarily seem to be enough to guarantee victory: She is a woman. But the Clintons have one supreme disadvantage in Democratic terms: They are white.

Look at it from the standpoint of the so-called "superdelegates," who, as the race narrowed down with the two contenders so evenly matched, had the delicate task of choosing between them. Was there really any serious possibility that they would reject the black contender?

Many people simply don't realize how powerful the African-American influence is in the Democratic Party today. In a Democratic national convention of 2004, about one-fifth of the delegates were black. A greater percentage is expected this year. And in the voting booths on Election Day, at least three-quarters of African-American voters are expected to cast their ballots for the Democratic candidates. A Democratic victory at the presidential level, and at most others, is simply inconceivable without that overwhelming margin among blacks.

How likely was it, then, that the national leadership of the Democratic Party, faced with a choice between roughly equally qualified contenders -- both United States senators with relatively short records in public office -- would turn down the African-American and nominate the white? What would millions of black voters, who have supported the Democratic Party faithfully all their lives, have thought of such a decision? How would they have reacted? The question almost answers itself.

The scene now shifts to the general election. Here, the dynamics are very different. Only about 10 percent of those who vote in a presidential election are African-American. And it is far from certain that other minority blocs -- most notably the large number of Hispanic-Americans who are not themselves black -- would choose a black nominee over an equally qualified white.

It is certainly true that America has made spectacular progress in diminishing the streak of racism that used to typify white voters. Most white Americans today, I venture to say, would be ashamed to vote against an otherwise attractive candidate because he or she is black. But there are, undeniably, some who would -- not that they would necessarily admit this to a pollster or report their vote accurately after they cast it. The question is, how big is this group? And is it bigger or smaller than the number of whites who would actually vote for an African-American precisely as a way of affirming that race should play no part in our politics? There are many such people, and their impact must not be discounted.

Karl Rove, the shrewd Republican operative whose estimates deserve high respect, recently said publicly that, in his opinion, the nomination of a black candidate like Obama would actually net more white votes for the Democrats, on the basis of his race, than it would lose.

There are, of course -- and ought to be -- other considerations. One, surely, is the other qualifications of the rival candidates. Obama has spent four years in the U.S. Senate (his only federal office); McCain has spent 22, preceded by four in the House of Representatives. Should that matter, and, if so, how much? McCain is an injured war veteran who spent five years as a prisoner of war in Vietnam; Obama has no military record. People who rightly insist that Obama's race is irrelevant will have a harder time saying the same about these remarkable facts.

The case for McCain is powerful and has nothing to do with race.