Obama comes out swinging -- gently - Edwards is more the aggressor in taking on Clinton at the debate.

Under fire from the first question -- an invitation for Obama to take a shot at Clinton -- the New York senator smiled through most of the two-hour session, often seconding the views of others on stage and joining the laughter during an attack on Republican Rudolph W. Giuliani.

The sharpest exchange centered on suggestions that Clinton was too divisive to win the White House. She said Republicans' focus on her candidacy showed -- "in a perverse way" -- its strength. "They obviously think that I am communicating effectively about what I will do as president," Clinton said.

Sen. Obama of Illinois responded moments later.

"Part of the reason that Republicans, I think, are obsessed with you, Hillary, is because that's a fight they're very comfortable having," he said. "It's the fight that we've been through since the '90s . . . and what we don't need is another eight years of bickering."

John Edwards, the aggressor throughout most of the evening, was even harsher. Noting Clinton's acceptance of campaign money from Washington lobbyists, the former North Carolina senator said, "Will she be the person who brings about change in this country? You know, I believe in Santa Claus. I believe in the Tooth Fairy, but I don't think that's going to happen. I really don't."

For the most part, however, the candidates spent the night rehashing old arguments over Iraq, Iran and Social Security. At one point, New Mexico Gov. Bill Richardson chided others for ganging up on Clinton.

"We need to be positive in this campaign," he said, as Clinton stood behind a nearby lectern, nodding in agreement. "Yes, we need to point out our differences . . . but I think it's important we save our ammunition for the Republicans."

The session at Drexel University in Philadelphia -- the seventh time the Democratic hopefuls have shared a debate stage -- came at a significant time in the presidential race, which appears split into parallel contests: one taking place in Iowa's caucuses and the other nationally.