PHILADELPHIA -- Sen. Barack Obama criss-crossed this city as though he were running for mayor yesterday, nurturing what he hopes will be a massive voter turnout just over three weeks from now.

"This is a pretty good turnout," Mr. Obama said, without fear of contradiction as he gazed over a crowd estimated at 20,000 that stretched for blocks down 52nd Street in West Philadelphia. "We have just had a spectacular day today."

It was largest of four successive rallies, under brilliant blue skies, in which the Illinois Democrat drew big crowds in neighborhoods throughout a city essential to his hopes of capturing the state. If successful -- and state polls are tilting increasingly in Mr. Obama's favor -- his efforts would frustrate a major expenditure of time and money by Sen. John McCain.

Pennsylvania is the largest state carried by Sen. John F. Kerry in 2004 that is seriously targeted by the McCain camp. The GOP nominee has spent almost twice as much time in the state -- 16 appearances to Mr. Obama's eight before yesterday's quartet -- than the Democrat. For much of the campaign, Mr. McCain had also led in television spending in the state, although a recent upsurge in Mr. Obama's commercials has erased that edge.

Mr. Obama has made a significantly heavier investment in the state in grass-roots organizing and, particularly, in voter registration. When Mr. Kerry won the state four years ago, the Democrats had a voter registration edge of roughly 600,000. Now, in large measure due to the Obama candidacy, the advantage has swollen to nearly 1.2 million. But for those numbers to matter, the new voters have to show up on Nov. 4.

That was the message repeated by Gov. Ed Rendell, the former partisan of Sen. Hillary Rodham Clinton, at each of yesterday's stops. Noting that voter turnout in the city had been 53 percent during the primary, he said, "If we do 53 percent, we're going to lose," he said. "We have to do 70 percent, 75 percent. Can you do that? ... I don't care how long the lines are -- nobody leaves."

Back to his base

It was, in one sense, an atypical day for Mr. Obama compared to the pattern of his recent campaigning. The Illinois Democrat has spent more time in recent weeks in states carried by President Bush four years ago as his rising poll numbers have allowed him to widen his reach. Yesterday, in contrast, was a serial appeal to the core of the Democrats' Pennsylvania base.

As he spoke yesterday, Mr. Obama once again assailed Mr. McCain's policies, but he also made a point of praising his rival for cautioning Republican partisans against incendiary anti-Obama rhetoric. At a town hall meeting in Minnesota Thursday, Mr. McCain had emphasized that Mr. Obama was a "decent man," and a "family man," in response to questioners who had urged him to go after the Democrat as a frightening, somehow foreign figure.

"I want to acknowledge that Sen. McCain tried to tone down the rhetoric yesterday, and I appreciated his reminder that we can disagree while still being respectful of each other. I've said it before, and I'll say it again -- Sen. McCain has served this country with honor, and he deserves our thanks for that," Mr. Obama said in language that he repeated at each stop. "But when it comes to the economy, and what families here in Pennsylvania are going through, Sen. McCain still doesn't get it."

Pushed by Rendell

"I know these are difficult times," Mr. Obama said at another point in the speech that he repeated with minor variations at each stop. "I know folks are worried. But I also know that now is not the time for fear or panic. Now is the time for resolve and steady leadership -- because I know we can steer ourselves out of this crisis. This is a nation that has faced down war and depression, great challenges and great threats."

Tucker Bounds, a spokesman for the McCain campaign, dismissed Mr.Obama's call to "disagree without being disagreeable."

"The tone of this election is not fueling voter outrage," Mr. Bounds said in a statement. "It's that Americans are frustrated knowing that Barack Obama's plans to raise taxes during a down economy and his proposal for a trillion dollars in new government spending are the absolute wrong answers to our economic crisis."

At three of Mr. Obama's stops -- in North Philadelphia, Germantown and West Philadelphia -- he faced crowds that were heavily African-American.

In Progress Plaza in North Philadelphia, Demetrius Robinson, waited for the senator with his 19-month-old daughter Safa, held in one arm and his son, Jibril, 4, in the other.

"I'm voting for Barack Obama because I want the next president to be someone my children can relate to, someone who looks like their father," he said. "I want them to say, 'I can grow up to be president. I can aspire to be something great.' "

Outside the Mayfair Diner, in Northeast Philadelphia, however, the crowd was more heavily white, many carrying union signs, reflecting a working class community that voted heavily for Mrs. Clinton in April's Pennsylvania primary. Republicans concede that Mr. Obama will carry the city on Nov. 4, but these neighborhoods are among those in which they hope to score well enough to hold down the Democrat's margin.

Speaking to reporters at one stop, Mr. Rendell said yesterday's barnstorming would help prevent that from happening.

"He hadn't been in Philly since the rally right before the primary, and we felt it was important ... for him to come to the neighborhoods," said the former mayor. "The buzz is out," he added. "By tonight, everyone in Germantown is going to know that Sen. Obama was in Vernon Park.

"No question that [Mayor Michael Nutter] and I were clamoring to get him to Philadelphia, clamoring to do this type of day," he said. "We had a little bit of a tug with [Obama headquarters in] Chicago but I think they're pretty happy."

Sean Smith, spokesman for the Obama Pennsylvania campaign, disputed any suggestion of a tactical dispute between the campaign and local politicians. Emphasizing that they had been eager to get their candidate to Philadelphia, he said, "It was just a matter of finding time on the schedule," he said.