PHILADELPHIA -- As hundreds of students and perhaps thousands of volunteers descend upon the campus of Drexel University, top Democratic candidates John Edwards and Barack Obama find themselves painted into something of a corner. Each have promised their supporters, and themselves, that they will land blows against Hillary Clinton, increasingly running away with the Democratic nomination.

Whether they can actually do it, however, is another question. The opportunities for candidates to establish themselves as real contenders are waning, and if Obama or Edwards do not land serious body blows soon, their chances of a twelfth-round upper cut in Iowa fade.

Each candidate has important questions to answer tonight. For Obama, who telegraphed his punch in a weekend New York Times interview, buzz is growing that supporters of his are getting restless with his failure to shake the Clinton foundations. After two efforts to change the tone of the debate amounted to little more than false starts, Obama may be forced to take a swing, and for someone unaccustomed to direct verbal jabs, anything less than a perfect swipe only hands Clinton an opportunity for a counterpunch.

Obama's long dedication, as well, to what he calls the politics of hope provides him a delicate balancing act that seems to argue for an attack. While any overt confrontations Obama begins will be greeted by cries that he abandons pretenses of an all-positive campaign, if he decides to avoid direct clashes, Democratic primary voters may wonder how he would be able to stand up to broadsides from the eventual Republican nominee, none of whom will be shy about taking him on.

For Edwards, the risk is of becoming a shrill attack dog. Edwards has done the most to draw contrasts between himself and Clinton, but none have proven terribly effective. Refusing lobbyist money and promising to remove troops from Iraq were Edwards' issues, but his contrasts are muted by the fact that Obama shares many of his policies. Ironically, it may be Edwards' criticisms that makes Clinton vulnerable to another candidate, but because Obama is the fresh new face, Edwards may not be the beneficiary of his own hard work.

If Edwards' campaign is thinking the same thing, it may be more important for the former Senator to draw his contrasts with Obama and set himself up as the Anyone But Clinton candidate.

The second tier can do almost nothing to break through, as countless debates have already shown. In hopes of winning a prominent position in tomorrow's stories, one possibility is to come to Clinton's defense and contrast not with her, but with Obama and Edwards. On the other hand, second-tier candidates this year seem unlikely to be prominent vice presidential contenders, and their campaign staffs may decide that lobbing grenades at any front-runner, at any opportunity, is their best hope.

Going into tonight's debate, in the city of brotherly love, at least two top Democratic candidates are lacing up their gloves. It is appropriate that a statue of Rocky Balboa, the world's most famous fictional boxer, stands just a short distance away on the steps of the Philadelphia Art Museum.