The meeting will be the 21st debate of the Democratic nominating contest. It also may be the last.
Obama accepted an invitation to debate on Saturday in North Carolina. Clinton said she would debate there on April 27. Neither campaign has agreed on a date. North Carolina holds its primary May 6.
The Philadelphia debate came amid a heated back-and-forth between the two over Obama comments at a private San Francisco fundraiser in which he said residents of small towns cling to religion and guns out of bitterness over their economic plight.
Clinton has called the remarks elitist, while Obama said he chose the wrong words to express the economic insecurity many workers face. Both campaigns are running television ads in Pennsylvania that focus on the flap.
Reflecting the acrimonious tenor of the race, Clinton also criticizes Obama for accepting campaign contributions from oil company executives in a new ad. Obama answered Wednesday with a commercial in which he says he doaccept contributions from federal lobbyists and political action committees, as Clinton does.
With 10 contests remaining, Obama leads Clinton in the popular vote, pledged delegates and states won. He picked up the endorsements of three superdelegates on Wednesday -- Reps. Andre Carson of Indiana and Mel Watt and David Price of North Carolina. Indiana also holds its primary May 6.
Clinton hopes to raise doubts about Obama's electability in a matchup with Republican Sen. John McCain to persuade undecided superdelegates -- the elected officials and party leaders free to vote their preference -- to back her candidacy.
Polls show Clinton leading Obama in Pennsylvania, but the margin has narrowed significantly in recent weeks. The former first lady must score a decisive victory in the state to keep her candidacy alive.
ABC News is sponsoring and televising the debate, with Charles Gibson and George Stephanopoulos moderating.
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