According to The Associated Press tally, Obama leads Clinton in the delegate count, 1,414 to 1,250. At stake in Pennsylvania are 158 delegates.

A McClatchy survey released Sunday found Obama trailing her in Pennsylvania, 48% to 43%.

Fighting to pad her lead, Clinton Sunday ripped into her opponent for acting like a Republican.

"It's no wonder that my opponent has been so negative these last few days of the campaign," Clinton told a packed gym in Bethlehem, pointing to a health care ad from his campaign that she said opposes universal coverage.

"The last thing we need is to have somebody spending as much money as he has downgrading universal health care," she said. "That's what the Republicans do."

Her campaign also launched a TV ad accusing Obama of taking cash from lobbyists and corporations in spite of his pitch that he's not beholden to special interests.

"She's running ads now suggesting otherwise," Obama said in Reading. "In this campaign, I have not taken their money, and nobody can dispute that."

The campaign has spiraled deeper into the mud pit ever since Obama said that small-town Americans cling to guns and religion out of economic bitterness.

At the time, Obama was gaining on Clinton in Pennsylvania, and building a double-digit lead in national polls, which has since collapsed. The Clinton campaign pounded the issue relentlessly and tossed out a few more lines of attack recently, including Obama's association with a former member of the Weather Underground.

"She saw a hole and she's trying to drive through it," said Clay Richards of the Quinnipiac Polling Institute, who added that the back-and-forth has gotten Obama off his message and game plan.

Meanwhile, John McCain's latest financial reports show him in the best financial condition of his presidential drive with $11.6 million in the bank at the end of last month.

In a report filed Sunday, the Arizona senator says he raised $15.2 million in March, his top monthly fund-raising performance of the campaign.