Ground Zero in the Democratic presidential war is shifting again. This time the battlefield will be the city of Philadelphia.

Both Sen. Barack Obama and Sen. Hillary Clinton will make appearances in the city today as they continue to go mano a mano in the lead-up to the hotly contested state primary race that will be decided on April 22.

Clinton will be in Philadelphia this afternoon and will continue the theme she stressed yesterday in a major policy speech in Washington, D.C., that being her stance on the war in Iraq.

She’s trying to do an end-run on the debate surrounding her original 2002 vote backing the move to go to war in Iraq, and now stressing her role as one of the leaders in the “bring the troops home” as soon as possible brigade. The speech comes the day before the fifth anniversary of the Iraq invasion tomorrow.

But perhaps the spotlight will be brightest on Obama, who finds himself once again trying to put out a brush fire, this one caused by comments made by the minister of his church that have been widely interpreted as being anti-American.

Among the things the Rev. Jeremiah Wright Jr., a Philadelphia native, has proclaimed from the pulpit is that the United States in some ways brought the 9/11 terrorist attacks on itself; that the nation is somehow responsible for the AIDS problem; and that instead of “God Bless America,” he would suggest “God damn America” for some of its actions.

Obama has repudiated the statements, saying he rejects them completely. That hasn’t made them go away.

Today’s speech is being touted as a major foray into the idea of race and politics.

So here we are again. For those of you who have not noticed, Obama is black; Clinton is a woman.

As much as we try to get away from it, just like Al Pacino in the finale of “The Godfather,” “it draws us back in.”

I don’t especially care for what Wright said. But I don’t hold Obama responsible for that. I am a Roman Catholic. That does not mean I agree with everything I hear in the homily every Sunday.

Obama is smart enough to know that this could be a defining moment for his campaign. It’s basic stuff. People who are already uncomfortable talking about race seize on this kind of red-hot rhetoric like a caged tiger going after red meat.

Now he has to put out the flames. Polls indicate his ratings took a 5-point dip in recent days.

The heat is on. One of the more widely circulated complaints about Obama’s candidacy is that his resume is a tad thin, he doesn’t have the experience, and that he’s never been in the kind of positions the presidency presents as the most powerful person on the face of the earth.

Today could tell us a lot about Obama. The fact that he’s making this speech already tells us a lot about the rest of us.