LANCASTER -- Illinois Sen. Barack Obama chugged into the waning days of the Pennsylvania primary campaign yesterday on a fast-paced whistle-stop tour, speaking, at times from the platform of the final car as the train crawled past waiting crowds in Bryn Mawr and Wayne.

The tour stopped for rallies in Wynnewood, Paoli, Downingtown and Lancaster, and was to end with a big, late-night rally on the Capitol steps in Harrisburg. Thousands cheered at each stop as the train moved westward into the center of the state from Philadelphia, Mr. Obama's strongest area of support and home to 35 percent of the state's registered Democrats.

"It's an absolutely critical area for Obama. If he can't win Philadelphia and its suburbs by a huge margin, he can't win the state," said G. Terry Madonna, director of the Franklin & Marshall College poll. "My estimation is he needs to win 75 percent of the vote in the city, and I think he will."

For Mr. Obama, the focus was on undecided voters who he hopes to woo with a final burst of energy.

"You do have a choice in this primary," he told a crowd of about 5,000 in Wynnewood yesterday. "I am not in this race to play the [political] game better; I am in this race to put an end to the game playing. ... I am not interested in becoming more in line with the Republicans and adopting their tactics; I want to get the Republicans to start sitting down with Democrats and to start solving problems for the American people."

Those problems include the economy, the war in Iraq, the cost of higher education and the lack of federal funding for schools, he said.

He heard of those problems first-hand yesterday morning as he campaigned door-to-door in the Mayfair section of Philadelphia, speaking to supporters, undecided voters and even visiting homes where "Hillary" signs dotted the lawns.

"Hi guys. I just came to say hi. Sorry to walk on your lawn," he said as he approached one family. "We've got an election on Tuesday, and I'd love to have your support."

To a man who described financial struggles, Mr. Obama said, "I'm trying to go after oil companies and stabilize gas prices. We've got a bill in the Senate but they're loading it up with all kinds of tax breaks."

Later in the day, he took a similar message to the masses and asked for their help in a voice that boomed through the Susquehanna Valley.

"This is our moment to declare independence from a government that is not working, and take it back for everybody in Pennsylvania and everybody all across America. We need that spirit more than ever because we are in a defining moment in our history," he told about 3,000 people in Lancaster. "We need to move this country forward. That's what the American people are looking for."

Also yesterday, in an interview with the Post-Gazette, Mr. Obama indicated he might renege on a campaign spending agreement if spending by third-party committees isn't limited, too.

Both Mr. Obama and presumptive Republican nominee John McCain had previously told the Federal Elections Commission they would accept $85 million in public money instead of raising private dollars to finance their general election campaigns.

Now that Mr. Obama has raised more than $193 million to Mr. McCain's $66 million, he says he will follow through on that pledge only if there are assurances that political action committees and others follow the spirit of the agreement, too.

He pointed to the Swift Boat Veterans for Truth's involvement in the 2000 election. The group ran commercials attacking Sen. John F. Kerry, and its leaders donated millions to President Bush's campaign.

"I don't want Democrats to heavily disarm if we can't create a system where outside parties and third parties run Swift Boat ads, outspending the Democratic nominee," Mr. Obama said as the train made its way from Wynnewood to Paoli. "That's not something I would agree to. I want to make sure everybody plays by the same rules."

He insisted he has not changed his position.

"What I said very early on is that I would sit down with my Republican opponent and try to find a way to preserve public financing. I was the one who asked whether this was an option," he said. "If it wasn't for my queries to the FEC, it wouldn't even be an option for anyone."

Obama surrogates were out in full force yesterday, campaigning independently and joining the senator onstage at trackside rallies.

"This is the last weekend we can play a critical role. There are still people we can reach," U.S. Sen. Bob Casey Jr. said in an interview. "People are listening. They're taking this decision very seriously. ... Due to the strength of the two candidates and due to the condition the country is in, we're not just choosing a nominee. We're choosing a president in this primary."

Mrs. Clinton and her surrogates were also trying to visit as many areas of the state as possible in the most crucial 72 hours of the primary cycle.