Clinton criticized Obama on Sunday for including John McCain when saying all three presidential candidates would be better for the country than President Bush.
"We need a nominee who will take on John McCain, not cheer on John McCain," she said at a rally in Johnstown.
Obama had said: "Either Democrat would be better than John McCain ... and all three of us would be better than George Bush."
The two traded barbs over each other's tactics, continuing an escalation in tone since Saturday.
Tuesday's vote is the first nominating contest in six weeks and has become a major test in the Democrats' protracted fight.
Obama attacks Clinton's attacks
At a town hall meeting in Reading, Obama said Clinton believed in "the say-anything, do-anything, special interest-driven politics of Washington -- that that's how it's got to be, that that's how the game is played."
"I think we've got to change the tone of our politics," he said. "Our campaign is not perfect. There have been times when ... if you get elbowed enough, you start elbowing back."
Clinton said it was Obama who had gone negative since their Philadelphia debate last week.
"It's no wonder that my opponent has been so negative these last few days of the campaign because I think you saw ... a big difference between us," she said at a rally in Bethlehem.
"While my opponent says one thing, his campaign, he does another. You can count on me to tell you what I will do," she said in Johnstown.
Clinton, who with her husband former President Bill Clinton has been the subject of many conservative investigations since the couple first entered the White House in 1993, was endorsed on Sunday by the Pittsburgh Tribune-Review whose publisher, Richard Mellon Scaife, funded many of those probes.
"Clinton's decision to sit down with the Trib (editorial board) was courageous, given our long-standing criticism of her," the paper said. "Political courage is essential in a president. Clinton has demonstrated it. Obama has not."
Obama picked up an endorsement, too, from the Financial Times. "After Tuesday's vote, the Democrats should move quickly to affirm Mr. Obama's nomination," it said. "He is, in fact, the better candidate."
Ahead of Tuesday's Pennsylvania vote, most analysts believed Clinton would win but the size of a victory has become the focus of both campaigns.
Pennsylvania Gov. Edward Rendell, a Clinton backer, said Obama outspent her by a 3.5-to-1 margin, including more than $2.9 million on television ads in the state last week.
"That's almost obscene," Rendell said on CBS' "Face the Nation." "So to win by four, five, six, seven points under that type of spending differential, that would be a huge and very significant victory."
‘The math is very unforgiving’
Obama's camp responded that Clinton would have to win by much bigger margins in 10 remaining contests if she is going to catch Obama in the hunt for the 2,024 delegates needed to win the nomination at the party's national convention in August.
"The math is very unforgiving at this point when it comes to delegate counts, and that's what it's all about," said Sen. Richard Durbin, who supports Obama.
McCain was heading off on a multistate tour of areas hard hit by poverty. Before leaving, he addressed the issue of his temper, the was the subject of a front-page Washington Post story on Sunday.
He said on ABC's "This Week" that the examples in the story were decades old, "totally untrue or grossly exaggerated."
"I am very happy to be a passionate man," he said. "I love this country. I love what we stand for and believe in, and many times I deal passionately when I find things that are not in the best interests of the American people."