That drew a feisty rebuke from Clinton, who said a McCain presidency would continue the policies of the Bush administration.
"We need a nominee who will take on John McCain, not cheer on John McCain," she told supporters at a Johnstown rally.
With the Republican nomination long since secure, McCain criticized both Democrats for advocating higher taxes that he said would worsen any recession.
But he seemed more eager to criticize the front-runner, Obama, more than the former first lady. Obama's relationship with former 1960s radical William Ayers is "an open question," McCain said on ABC's "This Week." Without being asked, he said Obama had become friends with Ayers and "spent time with him while the guy was unrepentant over his activities as a member of a terrorist organization."
Ayers, an education professor, has been quoted in an interview as saying, "I don't regret setting bombs" decades ago. Obama has said Ayers lives in his Chicago neighborhood, but that they do not speak regularly.
Two days before the Pennsylvania primary with 158 delegates at stake, Obama and Clinton observed the rituals of Sunday campaigning: a visit to church, stops at restaurants catering to families, as many public events as possible.
And blanketing the state with attack ads.
"In the last 10 years Barack Obama has taken almost $2 million from lobbyists, corporations and PACs. The head of his New Hampshire campaign is a drug company lobbyist, in Indiana an energy lobbyist, a casino lobbyist in Nevada," said a new Clinton commercial airing in the campaign's final days.
If anything, Obama upped the ante with his rebuttal. His ad said he "doesn't take money from special interest PACs or Washington lobbyists - not one dime." Clinton does, it added, and accused her of "eleventh-hour smears paid for by lobbyist money ...."
Preprimary polls show Clinton with a lead in the state she must win to sustain her candidacy.
Overall, Obama has 1,646 delegates to 1,508 for Clinton in the Associated Press' count, with 2,025 needed to clinch the nomination.
He also leads Clinton by 1,414-1,250 among delegates won in primaries and caucuses, while she has the advantage, 258-232, among superdelegates, the party officials who attend the convention by virtue of their positions.
More than 300 superdelegates remain publicly uncommitted and, in interviews, many told the AP they want a candidate who can capture the White House. At the same time, others said that before deciding which contender to support, they will give special weight to the candidate with the most delegates won in primaries or caucuses, or the one who won their state or congressional district.
Even Clinton's allies concede she must win the Pennsylvania primary, and some have suggested she needs a sizable victory if she is to have a chance of overtaking Obama. But as was the case in other states, a strong popular vote win would not necessarily translate into a major gain in delegates for the former first lady.
Democrats award delegates proportionally based on the vote in congressional districts, and some have more than others. A congressional district in Philadelphia with a large black population, for example, has nine delegates, more than any other. Numerous other districts have four delegates, and the likeliest outcome in most if not all is for a 2-2 split.
Pennsylvania's primary marks the end of a six-week intermission in the Democratic campaign season. Only seven other states, Guam and Puerto Rico will follow over the next six weeks.
Partners in a historic race, Obama and Clinton squabbled through a final weekend of campaigning.
In Bethlehem - of all places - Clinton said the Obama campaign had abandoned its pledge to remain positive and stick to issues.
"While my opponent says one thing and his campaign does another, you can count on me to tell you where I stand and you can count on me to tell you very specifically the solutions I'm offering for America," she said.
Obama seemed uninterested in backing down.
"I disagree with Senator Clinton that lobbyists are part of the system," he said. "I think they are part of the problem."
Obama said he hasn't taken money from federal lobbyists and scolded Clinton for suggesting he has.
"She's running ads now suggesting otherwise. In this campaign, I have not taken their money, and nobody can dispute that," Obama said.
The Illinois senator also was running a commercial critical of Clinton's health care proposals in what his aides said was a response to an ad aired by an independent group that supports the former first lady.
Also, Obama aired a commercial that said that every major newspaper in Pennsylvania had endorsed him. "The (Pittsburgh) Post-Gazette calls Hillary Clinton's attacks the 'cynical responses of old politics,'" it said.
Ironically, Clinton drew an endorsement from the Pittsburgh Tribune-Review, owned and published by Richard Mellon Scaife. The billionaire conservative personally funded many of the Republican investigations that plagued Bill Clinton while he was president.
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David Espo reported from Washington. Associated Press writer Liz Sidoti in Reading, Pa., contributed to this report.
