No matter how big the circus, when it comes to town someone has to put up the tent.

And that's what was going on yesterday on the fourth floor at 1500 Sansom St., where in newly rented offices Sen. Barack Obama's presidential campaign began building in earnest for a suddenly important April 22 Pennsylvania primary showdown with Sen. Hillary Rodham Clinton.

Obama's organization is behind in the polls here - a Quinnipiac University poll released last week showed Clinton with 49 percent of the Pennsylvania Democratic vote and Obama with 43 percent - and faces Clinton's formidable political backing by Gov. Rendell, Mayor Nutter, and significant parts of the state's Democratic Party.

Until this week, many thought Pennsylvania would not be necessary. But Clinton revived her flagging campaign Tuesday with victories in the Texas and Ohio primaries. Pennsylvania, with its relatively late primary, became relevant.

So Obama's organization has begun pushing full-bore here, recruiting volunteers and - most important in Pennsylvania's closed primary system - registering as many Democratic voters as possible before the March 24 deadline.

Sean Smith, 37, the Connecticut political communications specialist brought in by the Obama campaign to be its Pennsylvania press secretary, was undaunted.

Smith said that in the last week Obama had opened five campaign offices in Pennsylvania - in Philadelphia, Doylestown, Bethlehem, Harrisburg and Pittsburgh - and that more would follow.

Smith said he was among 20 paid campaign staffers in Pennsylvania but "we'll probably get 100 more this weekend, and this might just be the beginning."

Over the weekend, Obama campaign staffers trained 2,000 Pennsylvania volunteers to help in telephone fund-raising and voter registration, which Smith said was where the campaign was now focusing its efforts.

Because of Pennsylvania's closed primary, only voters who are registered Democrats by the March 24 deadline can vote to nominate Obama, 46, of Chicago, or Clinton, 60, of New York, as the Democratic presidential candidate.

Smith said that 47,000 Pennsylvanians had switched parties to register as Democrats and that he believed the "vast majority" of them did so to vote for Obama.

"We can win with the people who are Democrats right now, but we just want to get everybody who does want to vote the opportunity to do that," Smith added.

Pennsylvania's voter rolls show 3.89 million registered Democrats, 3.25 million Republicans, and 984,349 independents and other-party voters. It is this latter group, Smith said, that Obama's volunteers will be pursuing over the next two weeks.

Smith noted that even before Obama began establishing his formal campaign organization here this week, volunteers had already achieved much. He said Obama's volunteers, including 25 Pennsylvania college campus chapters, had helped the candidate obtain 21,000 valid signatures in three weeks and more than 50,000 overall - just 2,000 were needed - to get Obama and a full slate of delegates on the primary ballot.

Obama leads Clinton in Democratic convention delegates, 1,569 to 1,462, according to the Associated Press, and most observers say Clinton needs the 158 Pennsylvania delegates at stake next month.

"We have found that she starts off with big leads within the political establishment and in the polls, but we have also found that in some states we have been able to overcome those advantages she begins the contest with," Smith said. "And we have won more caucuses, more delegates and more votes than she has."

Smith spoke yesterday from Obama's state campaign headquarters, opened Sunday in Philadelphia and still very much a work in progress.

At times Obama's Center City offices had the air of students preparing to mount a school play. Hand-painted signs graced windows with Obama keywords Change and Hope. Large hand-lettered banners proclaiming "Yes we can!" hung from the ceiling.

But if the furniture was sparse and usually of metal or bright plastic, and the phone system still being installed, it was not deterring a steady parade of citizens who came in to volunteer.

"So what do you know, Pennsylvania counts!" one woman exulted as she boarded the elevator with others to sign up to volunteer.

"I like his ideas," said Robert Gales, 80, of Fairmount, as he leaned on a cane near a lobby table where Obama volunteers directed people arriving to the elevator to the fourth floor. "I think, frankly, that he's a little more open to ideas and a lot more open-thinking."

Gales, a retired publishing-industry sales and marketing worker, described himself as a veteran political campaign volunteer who began 60 years ago in Harry S. Truman's campaign.

"I love his politics," added Deborah Williams, 56, who was staffing the lobby table. "I'm fascinated by the prospect of real change. . . . We need to get away from the stereotype politics, and especially the dirty tricks."

Williams said she had been laid off from her job as a customer-service representative for a local health insurer. She said she planned to return to school in April to brush up on her computer skills, "but until then this is my full-time job."

Beyond the campaign, grassroots groups have also sprouted, including Obama Works, which last Saturday sponsored a street cleanup of Point Breeze and the neighborhood around Graduate Hospital.

The group, which also registers voters, will return to Philadelphia at noon tomorrow to clean up the Chew Playground at 19th Street and Washington Avenue in South Philadelphia.

On the fourth floor, a diverse group of about 25 volunteers worked phone banks targeting various Philadelphia neighborhoods as prospective campaign workers gathered at the door preparing to sign up.

"We're working on blocking and tackling first before the quarterback does his thing," said Smith, adding that he did not expect to see Obama campaign in Pennsylvania before Tuesday's primary election in Mississippi. Obama and Clinton will also be seeking support tomorrow at Democratic caucuses in Wyoming.

Smith said he last worked for the John Kerry presidential campaign in 2004 and joined the Obama campaign "because I would like to win this year and I believe Sen. Obama has the best chance of beating John McCain," who clinched the Republican nomination this week.

Smith also said he believed Obama was "offering the country a chance to break this stalemate that has marked so much of . . . the last 10 to 15 years."

Despite Clinton's endorsements by Rendell and Nutter, Obama is being supported by U.S. Rep. Chaka Fattah, a Philadelphia Democrat and former mayoral candidate, and by U.S. Rep. Patrick Murphy, a first-term Democrat from Bucks County, along with what Smith called a "growing number of state representatives and mayors."

Still, the divide among Pennsylvania Democrats was apparent even among Obama's volunteers yesterday.

Williams, for example, is supporting Obama, but her son is backing Clinton.

"We're still working on him," she joked.

And volunteer Lucia Williams, 60, of Mount Airy, a retired city human-services worker and no relation to Deborah Williams, said her 23-year-old daughter was a Clinton supporter, mostly because she wanted to see a woman elected president.

Williams said she was backing Obama because she believed he was not using divisiveness in his campaign and because "the baby boomers have had their chance. We really need to bring in a new generation of leadership."