Not Rudy Giuliani, who is a supporter of abortion rights.

Not Tom Tancredo, who is a hardliner on immigration.

Not John Edwards, who is a critic of the war in Iraq.

Only Barack Obama, who is black.

No other presidential candidate, no matter his or her polarizing positions, has felt it necessary to seek protection from the Secret Service. But last week we learned that Obama has sought and will receive that protection, the only candidate ever to do so this early in the process. Only one other candidate even has a Secret Service detail: Hillary Clinton. And that's because she's a former first lady.

You know who else required early protection? Jesse Jackson, when he ran for president in '84 and '88.

Neither Obama's campaign nor the Secret Service will comment on precisely what went into the decision to assign a detail to the senator, beyond saying it was based on no one specific threat. But one need not be a seer to divine the reason. Put it this way: the darker the candidate's skin and the more serious his candidacy, the earlier he seems to need protecting.

All of which adds a telling dimension to the ongoing debate about Obama and blackness that has percolated for months beneath the surface of his candidacy.

On the one side, you have earnest white people insisting that, because his mother was white, Obama is not really black, but "biracial."

On the other side, you have earnest black people insisting that, because his heritage does not trace to slavery, Obama is not really black enough — that is, not black in a cultural sense.

Apparently, however, he is both black and black enough for whatever individual or individuals unnerved his handlers enough to seek Secret Service protection. That's a truth that cuts the clutter.

In a sense, the fact that we have the luxury of debating "what" Obama is testifies to the racial progress this nation has made. Once upon a time, nobody had to debate. Back before Colin and Cosby and Condoleezza, before Air Jordan took wing and Johnson made Magic, before Oprah was America's favorite sister girl and Martin spoke of dreams, back when a Southern restaurant caused an international incident by refusing service to an African diplomat ... back in the day, there was no need of abstract rhetoric on what black is.

You knew. The world made sure of it.

If we have moved beyond that day, if we are proud to think ourselves more enlightened now, it is nevertheless naive to believe the naked meanness of that day has wholly disappeared.

It is fashionable now to speak of systemic racism and the need for black folk to take a greater hand in their own salvation. Those discussions are valid. But it is also occasionally instructive to remember that old-fashioned mean-as-a-snake, thick-as-a-brick hatred is still alive and well and living in the USA.

Sometimes, it lolls in the shade of the intellectual cover provided it by the likes of Rush Limbaugh.

Sometimes, it is dressed in suit and tie and told to sound reasonable by the likes of David Duke.

Sometimes, it is sung in wobbly adolescent voices by the likes of Prussian Blue.

And sometimes, it just rears up on its hind legs and brays that it will commit violence rather than accept a black man as its president.

We like to pretend this bile is not still in us. We like to pretend we are beyond it. Then the man who could be our next president must ask to be protected from those who think him too dark for the job.

Something to remember next time you are tempted to debate what black is. The world still has ways of making you know.

Leonard Pitts Jr., winner of the 2004 Pulitzer Prize for commentary, is a columnist for the Miami Herald. Reach him at lpitts@miamiherald.com.