Though modest compared to the hundreds of thousands who greeted him in Berlin last week, the crowd of 35,000 who gathered on an April night on Philadelphia's Independence Mall to listen to Sen. Barack Obama was one of the largest crowds in the state's political history.

Despite that enthusiasm, the scores of thousands of new Pennsylvania voters registered by his campaign and his huge spending advantage, Mr. Obama was clobbered in April by Sen. Hillary Rodham Clinton.

Months later, the Obama campaign believes that spring setback sowed seeds for a November harvest. Indeed, long-term demographic trends and more recent polls and party registration shifts suggest that in November the state is Mr. Obama's to lose.

"The primary allowed us to build an infrastructure that is enormously helpful going forward," said Craig Schirmer, the Democrat's freshly installed state director. "Having the experience of such a competitive primary gives us a big advantage over John McCain."

A Clinton partisan then, but an Obama ally now, Gov. Ed Rendell suggests that the Obama campaign's grass roots efforts may have paid off in ways that were not immediately recognized in the wake of Mr. Obama's nine-point primary loss.

"They do the registration and field stuff very, very well," he said. "Had they not done the registration and the field as well as they did, I think we would have beaten them by 17, 18, 19 points."

According to the Department of State, 218,923 new voters registered in Pennsylvania before the primary -- 152,775 Democratic and 40,195 Republican. Another 164,026 voters switched their affiliation to the Democrats, compared to just 14,887 who changed to Republican.

Mr. Rendell estimated that those new Democrats favored Mr. Obama by more than two-to-one. And, he said, "As much as it was expanded in the run-up to the primary, I still believe there are anywhere from 50,000 to 100,000 new Democrats more to be added... I'm hoping we'll see another 100,000 new Democratic voters by the time the registration closes."

That primary may have laid a grass-roots foundation for the campaign, but it also underscored some of the challenges Mr. Obama faces as he tries to corral the state's electoral votes. Chief among them is his need to court the working-class voters who strongly supported Mrs. Clinton.

In the primary campaign, Mrs. Clinton's partisans pointed to such results to sustain their argument that Mr. Obama's failures to win major states such as Pennsylvania, Ohio and California showed his vulnerability in the fall. The Obama camp has always branded that "big-state" analysis a political fallacy, a comparison of spring apples to autumn oranges.

"The general election here is a completely new ballgame," said Mr. Schirmer. "It isn't a choice between two strong Democratic candidates."

Mr. Rendell sees intra-party healing as a work in progress but a manageable one.

"The other thing he has to do is bring back parts of the Democratic base that I don't think is 100 percent there yet -- and that would be some women, although I think most women are returning, but there might be some Hillary supporters still out there. And you have to pay particular attention to blue-collar Democrats in the northeast and southwestern Pennsylvania," he said.

Brad Coker, managing director of Mason-Dixon Polling & Research, agreed, though he threw the Erie area into the mix as well. "In the last battle [in the primaries], those blue-collar voters went for Hillary. Obama did not do well with them.... Those more culturally conservative Democrats need to be sold on Obama."

The Obama campaign hopes that voters such as county Democratic chair Tammie Shetler, of Indiana County, which was carried overwhelmingly by Mrs. Clinton in April, will be typical of the former Clinton cadres.

"It took me a while, I have to tell you," she said. "I had to kind of sit back for a while, but I do think Sen. Obama has a lot to offer this country ... the big picture for the working class right now is the economy. It's really killing everyone."

The Democratic candidate complicated that challenge for himself and his campaign with his musings about "bitter" voters who "cling" to their religion and guns -- remarks first reported during the Pennsylvania primary and seized on by the Clinton campaign.

Mr. Schirmer said the campaign was ready for that challenge.

"We're going to be reaching out to rural voters, to voters all over the state .... voters here are going to find out more about Barack's life story and find that his struggles were like their own," said Mr. Schirmer.

"He breaks through on pocket-book issues," Mr. Rendell predicted. "The good news for Sen. Obama is at times when the economy isn't necessarily strong, people care about pocketbook issues rather than niche issues -- gay marriage, abortion."

Another region where Mrs. Clinton did well was suburban Philadelphia. Those four "collar" counties, once a Republican stronghold, have been tending toward the Democrats in state and national elections. McCain campaign officials have said they hope to persuade many of those voters to return to the region's GOP roots.

Mason-Dixon's Mr. Coker thinks that will be a tall order.

"Hillary ran pretty well there in the primary, but I still think that will ultimately be Obama country," Mr. Coker said. "A lot of the suburbs in the Northeast have changed," he added. "The voters are a little younger, there's more problems that you used to associate with urban areas.... I just think the suburban voters are not as culturally conservative as the Republican Party has become in recent years."

The Democratic drift of the southeastern suburbs is a trend that predates the Obama campaign, but it's also one that the campaign reinforced. In the last six months, the registration edge in both Montgomery and Bucks County has shifted to the Democrats, and the Obama operatives are widely credited for that turnabout.

"When I ran just last year, the Democrats trailed in registration by 30,000," said Montgomery County Commissioner Joe Hoeffel, a Democrat who supported Mrs. Clinton in April. "Now Montgomery is Democratic by 15,000. In large measure that's because of the primary and I think most of that new registration was for Obama."

Of a region that's long been part of his political base, Mr. Rendell said, "The question is not whether [Mr. Obama] will win the suburbs -- I think he'll carry the suburbs -- but the key to the margin is getting the Clinton supporters, in particular Clinton feminist supporters on board. If he does, and I think he will, he can do as well as Sen. [John F.] Kerry did in the suburbs and maybe better."

Another obvious focus of the registration and turnout push will be Philadelphia itself. The city represents a core constituency area for any Democrat running statewide. Mr. Obama carried it easily in the spring primary. With its significant black population, it holds obvious continuing importance for his November hopes.

"He'll do as well or maybe better than John Kerry did in the city, if he does, that almost represents an insurmountable lead in the state," Mr. Rendell said.

The governor, who campaigned relentlessly for Mrs. Clinton in the spring, said he's ready to make a similar effort for the soon-to-be nominee.

Mr. Schirmer depicted the spring registration push as a preview of coming attractions. He estimated that the campaign had been responsible for roughly 150,000 new Democratic registrations before the pre-primary deadline.

"We don't give away our goals but it's safe to say that registration remains an important part of our program," he said.

And that program is ambitious. Fueled by the financial advantage that the campaign anticipates as a result of its decision not to accept federal funding for the general election, the Obama forces predict that they will have a significant advantage at the grass roots -- "a plan that will be unprecedented in scope and ferociously executed," is the way that Mr. Schirmer describes it.

The campaign announced last week that it had already opened 24 offices in the state. The Obama operative said that their structure would be "larger than anything anyone has ever done here in the general election."

This is Mr. Schirmer's first Pennsylvania campaign, but he has plenty of perspective on grass roots campaigning. In the 18 months that he's been with the Obama effort -- "an unbelievable ride," he said -- he's played key roles in three decisive victories.

He was get-out-the-vote director in South Carolina, where Mr. Obama, abetted by one of the record turnouts that became routine as the primary season went on, won by 28 points over Mrs. Clinton. He was state director in Wisconsin, where Mr. Obama prevailed, 58 percent to 41 percent. He was also the chief on-the-ground operative for the campaign's 56 percent to 42 percent win in North Carolina, a victory that all but locked up the nomination for the Illinois senator.

Mr. Schirmer described himself as "very metrics-based" in his approach to any state's campaign.

"Grass roots politics is an art, but it's also a science," he said.

"It is the voter registrations. It is the number of doors we knock on, the number of phone calls we make. All of those are things I want to know about daily ... Mechanically, it's incredibly tedious, but that's how you win elections."