PHILADELPHIA -- In a series of stops that melded high-flown rhetoric to rock-concert ambience, Illinois Sen. Barack Obama yesterday entered the final weekend of a contentious Pennsylvania primary campaign, stressing his keynote theme of changing the political culture.

The background strategy, according to campaign insiders, is to stay positive in the face of a late round of attacks by New York Sen. Hillary Rodham Clinton, in hopes that she will drive undecided voters into the Obama camp.

Mr. Obama's sole Clinton references were aimed to etch a contrast between her as an exemplar of old-fashioned politics and himself as a harbinger of change. That strategy was on display in two stops on opposite ends of the state yesterday, as he rallied supporters at an invitation-only "town hall" meeting at the Penn State Behrend Campus in Erie, then in a mass open-air rally at Independence Mall in this city, where hip-hop artist Will.i.am and Ed Kowalczyk of the York, Pa., alternative rock band Live were his opening acts.

By the Obama campaign's estimate, more than 30,000 people crowded onto the Independence Mall lawn and spilled into surrounding streets and sidewalks at the Philadelphia event.

In his remarks yesterday, Mr. Obama sketched a tableau of unfulfilled political promises, from energy independence to health care and economic development -- promises he says date to the 1970s.

"We're still talking about it in 2008. And everyone here knows why. Because in every election, politicians come to your cities and your towns, and they tell you what you want to hear, and they make big promises, and they lay out all these plans and policies. But then they go back to Washington when the campaign's over, and nothing changes," he said in his Erie appearance.

"This year, we can't afford the same old politics. This year, we can declare our independence from this kind of politics. That's change we need right now," he said.

More than 2,500 partisans jammed into the Junker Center at the Penn State campus in Erie and heard Mr. Obama put in a good word for impatience. "We can't wait to fix our schools. We can't wait to fix our health care system. We can't wait to bring good jobs here to Erie, Pennsylvania -- we can't wait," he declared.

He was drowned out by chants of "We can't wait!"

His sole swipe at Mrs. Clinton was preceded by praise for her, then a charge that she remains mired in the old political style. "Senator Clinton is a tenacious opponent and a committed public servant," he told the crowd. "But her message comes down to this -- we can't really change the say-anything, do-anything, special-interest-driven game in Washington, so we might as well choose a candidate who really knows how to play it."

In both Philadelphia and Erie, Mr. Obama criticized Mrs. Clinton for accepting money from Washington lobbyists and referenced Clinton campaign attacks that criticized his attendance at a Chicago church whose then-pastor, the Rev. Jeremiah Wright, delivered incendiary critiques of U.S. policy, and Mr. Obama's own comments about "bitter" voters in this state.

The Clinton campaign lashed back in a phone interview late last night. "Mr. Obama's hypocrisy is stunning. What he says and the actions he takes are in two completely different places," said campaign spokesman Mark Nevins.

The Clinton spokesman said Mr. Obama complains of negative campaigning, then sends out anti-Clinton mailings; he says she takes money from lobbyists, yet he accepts money from people who run political action committees. "I'm going to have to throw a party and introduce Sen. Obama's actions to his words, because they've never met," Mr. Nevins said.

Mr. Obama left the harshest criticisms of Mrs. Clinton to his surrogates.

A top Obama aide, speaking on condition of anonymity, said the New York senator's high negative rating among voters -- a longstanding handicap for Mrs. Clinton -- was likely to rise in the wake of a pair of her ads attacking Mr. Obama. One features Pennsylvania voters decrying Mr. Obama's comment that some in this state "cling" to religion and gun rights as a reaction to hard economic times.

The Obama strategy of staying positive in the face of the ads was echoed by a leading supporter and adviser, former Pennsylvania Sen. Harris Wofford, who last night delivered his own withering critique of Mrs. Clinton's campaign while arguing that his candidate's forebearance could pay dividends among the estimated 6 percent of voters still undecided.

"My own sense is that she's bringing them in to us by turning people off with the negativity of her campaign," Mr. Wofford, a onetime ally of the Clintons during the battle for national health care, told the Post-Gazette in an interview last night. "I can't imagine any of the great presidents of the United States running a campaign like hers."

Mr. Wofford denounced "the negative, destructive and often-petty campaign to which, too many times, the former first lady has descended. Can you imagine Robert Kennedy conducting the kind of campaign Sen. Clinton is waging?" he asked.

Obama troops yesterday were preparing heavy get-out-the-vote drives as well as blanket field campaigning throughout the state over the weekend.

Sean Smith, state spokesman for the Obama campaign, said the candidate's final weekend before primary voting will include visits to regions where Mrs. Clinton has long been thought a prohibitive favorite, such as Scranton, Reading and the Lehigh Valley as well as a return swing through Western Pennsylvania.

Mr. Smith said the campaign has identified a wide range of potential supporters within the cadre of undecideds, and wooing them could provide the margin to hold down any Clinton advantage. "Those voters live in every part of this state," he said. "If we're going to keep this [anticipated Clinton victory margin] in single digits, we've got to mine every vote we can."

They are targeting voters such as Alicia Tavani, a 20-year-old Gannon University student, and the only openly declared undecided voter found at yesterday's Erie town hall. "A lot of their stands are similar. I'm torn," she said. "I think they're both great candidates that could be awesome presidents."