It used to be that the conversation about the very difficult subject of race in America was best left to African-Americans, because only they have experienced the active or passive oppression that many whites cannot comprehend.

Let the Walter and Armstrong Williamses, the Thomas Sowells and Floyd Flakes, the Michael and Shelby Steeles, the Larry Elders and Ward Connerlys and Jay Parkers — the theory went — haggle it out with the Al Sharptons and Jesse Jacksons, the Carl Rowans and Julian Bonds, the Adam Clayton Powells and (of course) the Martin Luther Kings.

Then came two realizations — (1) the African-American community is as ideologically divided (between conservatives and liberals) as the white community, and (2) the McCain-Obama campaign is at hand. So now the discussion is open to all.

Rightly or wrongly and largely unspoken, race is a deep-running factor in American culture — infusing much that it should not but does. Barack Obama is the first African-American with a genuine prospect of becoming president of an electorate that is 11 percent black and 77 percent white. Because of that percentage discrepancy, Obama’s chances of winning depend greatly on the extent to which — in commentator Juan Williams’ words — he can “assure undecided white voters that he shares their (conservative social) values and is worthy of their trust.”

So how seemingly odd that Obama should inject race into the campaign. Possibly he did it to build a force field around him to deflect every criticism of every kind.

During the primaries, he blasted Bill Clinton for allegedly making race an issue in the Carolinas — implying Clinton was doing it to gin up white turnout for Hillary. Obama also perceived subtle racial undertones in John McCain’s first general-election ad — i.e., its description of McCain as “the American president Americans have been waiting for.”

In late June, Obama began mentioning his race (as he frequently had) in combination with dark implications that McCain would deploy race against Obama (as McCain never has): They’re going to try to make you afraid. They’re going to try to make you afraid of me. “He’s young and inexperienced and he’s got a funny name. And did I mention he’s black?”

Finally on July 31, in Springfield, Mo., Obama dealt down and dirty:

“Nobody really thinks that Bush or McCain have a real answer for the challenges we face. So what they’re going to try to do is make you scared of me. You know — he’s not patriotic enough. He’s got a funny name. You know, he doesn’t look like all those other presidents on those dollar bills. You know. He’s risky. That’s essentially the argument they’re making.” (Italics added.)

Properly fed up, the McCain campaign jumped on the “he doesn’t look like all those other presidents” comment. Said McCain’s campaign manager: “Barack Obama has played the race card, and he played it from the bottom of the deck. It’s divisive, negative, shameful, and wrong.” Said McCain himself of his campaign manager’s comment: “I agree with it, and I’m disappointed that Senator Obama would say the things he’s saying.”

Whereupon Obama’s campaign manager said the McCain campaign’s very mention of the “not like other presidents” remark, combined with a McCain ad depicting Paris Hilton and Britney Spears, are “character attacks.” He was echoed by New York Times columnist Bob Herbert, who sees a “venomous McCain campaign” all about “trashing the opposition, Karl Rove-style” — a campaign “smearing Mr. Obama every which way from sundown.”

This Kabuki invites the question: What is Obama’s rationale?

It’s racial politics, and goes like this: Because I’m a minority, you can’t use race but I can. If you do, I’ll zap you. And if you don’t and I do and you call me on it, I’ll zap you for even suggesting I’m playing racial games.

But racial politics is loaded with risk.

Obama needs to maximize minority turnout while pulling enough white votes to win. Yet the polls in this campaign are all over the place, meaning there’s no telling how Obama’s racial games will play out.

Many sufferers of white guilt want desperately to vote for an African-American to prove to themselves — if to no one else — that they are not racist. Still, the extremist views of Obama’s preacher Jeremiah Wright, and now the playing of the race card by Obama himself, may combine to generate in guilt sufferers the sentiment, not this time.

Nor dare we forget ideology. Liberalism bears its own heavy racial-ethnic burdens: Liberal college administrators imposed Ivy League admission quotas on Asians and Jews. And liberals of various professional stripe turned Anita Hill in their efforts to stop the confirmation of the conservative Clarence Thomas, who writes eloquently (in “My Grandfather’s Son”) about the extent to which the leftist concept of affirmative action robs blacks of their dignity.

Obama’s shameless playing of the race card — his combining of race with liberal ideology — might prove his undoing in what remains a deeply conservative culture. It raises profound questions about his judgment and leadership, and may leave him in voters’ minds precisely the inexperienced, insubstantial, immature celebrity of John McCain’s description.