A vast army is about to engulf Pennsylvania as the Democratic race for president enters its next phase, and the advance guard yesterday afternoon was a slender young woman in jeans and a tweed jacket.

"I'm proud my mom stood up for universal health care before it was fashionable," Chelsea Clinton said, answering questions at the University of Pennsylvania.

The state became a battleground because Sen. Hillary Rodham Clinton, with her back to the wall, won three primaries over Sen. Barack Obama on Tuesday and made the state's April 22 primary relevant for the first time in a generation.

After contests Saturday in Wyoming and Tuesday in Mississippi, Pennsylvania will have the only game going for about six weeks.

In other words, look out.

"There's so much time that the candidates are not going to be just flying back and forth between Pittsburgh and Philadelphia," said David Sweet, a Harrisburg lawyer who was manager of Gov. Rendell's 2002 campaign and is now supporting Obama.

"It's going to get down to looking at polls to see if the western part of Washington County is in play, what parts of Allentown to visit, or even the upper vs. the lower portion of Dauphin County," Sweet said.

Jon Delano, professor of public policy and politics at Carnegie Mellon University, said about 40 percent of registered Democrats live outside the Pittsburgh and Philadelphia areas, so the candidates cannot ignore smaller cities and towns.

"It's going to be an absolute zoo," Delano said.

With 158 pledged delegates at stake, Pennsylvania is the largest remaining prize in the Democratic contest, with neither Obama nor Clinton close to the 2,025 delegates needed to win the nomination.

Already yesterday, the Clinton campaign had 25 paid staffers in the state, and the number will "grow exponentially," spokesman Mark Nevins said. Dozens of volunteers staffed phone banks for Clinton at the Philadelphia Federation of Teachers union hall as the campaign's headquarters was readied.

"We were up late last night and early this morning," said Leslie Miller, director of communications for Obama's Philadelphia campaign, which drew dozens of volunteers to its headquarters in Center City.

On paper, Pennsylvania seems tailor-made for Clinton, filled with the demographic groups that exit polls show have formed her base in states she has won: older voters, white ethnics, Catholics, and Democrats who make less than $50,000 a year and have less education.

"Where you have a working-class Democratic household where both the husband and wife are in bowling leagues and their last name is Horniak, that's her crowd," said Washington-based Democratic consultant Tom Pazzi, who is not working in the presidential race.

For demographic and other reasons, pollsters and analysts say Clinton enters the Pennsylvania battle as the favorite. Among her advantages: support from Rendell and large parts of his political network, and deep ties to Democratic leaders from her years as first lady. In addition, Clinton's late father was a native of Scranton, and she vacationed often at Lake Winola as a child.

Still, the state could swing either way, analysts say. After all, there are areas of potential strength for Obama: Philadelphia, with its large African American population, and the suburbs of the city, home to large numbers of more affluent, educated voters who have flocked to him in state after state.

"West of King of Prussia, I think he has a tougher time," said Philadelphia-based Democratic consultant Larry Ceisler. "The population gets older, more working-class, not as highly educated. But he has room for growth, and the time to convert people."

Ceisler believes that to win, Obama will have to make inroads in Southwestern Pennsylvania, the declining industrial areas around Pittsburgh.

The latest Quinnipiac University poll, released last week, showed that Clinton's lead over Obama had shrunk to 6 percentage points, from 16 points as recently as mid-February.

"It's Clinton's race to lose right now, but seven weeks is a long time, and a lot of things can happen," said Clay Richards, assistant director of the Quinnipiac University Polling Institute.

Pennsylvania voters overwhelmingly cited the economy as their number-one concern, with two-thirds believing that the country is already in a recession.

But the wars in Iraq and Afghanistan also resonate, Richards said: The state has a large number of men and women serving in the armed forces there, and a large population of veterans. And one of the Sept. 11 planes crashed in Pennsylvania.

"All those things have brought the issues of war and terrorism home," Richards said.

Clinton used concerns about Obama's relative inexperience on national security issues to help win in Texas and Ohio, the two key contests Tuesday.

Whatever issues emerge here, and despite the focus on smaller towns, the Philadelphia region will be a large battleground. Of the 103 delegates to be awarded based on the vote in congressional districts, 43 are from the six districts in and around the city.

If the congressional districts around Allentown, at the north end of the reach of the Philadelphia media market, are added, 52 delegates are up for grabs - just over half the total.

Only registered Democrats can vote in the primary, and there has been a huge uptick in registration changes in the last two months - more than 40,000, according to the secretary of state, and most are newly minted Democrats.

Sweet, the Obama supporter, said the campaign - and probably Clinton's too - aims to sign up as many independents as possible before the March 24 deadline.

"You have to have lots of volunteers, because it's very labor-intensive," Sweet said.

At Penn yesterday, the questions for Chelsea Clinton ranged widely. The first questioner asked what Hillary Clinton would do for gay, lesbian, bisexual and transgender Americans. Others asked about services for war veterans, universal health care, foreign trade, her views of the Bush administration, and human rights.

To most of the questions, she answered by beginning, "I'm proud that my mom . . ." and then launched into a detailed account of her mother's record and views of the topic.

Before long, Pennsylvanians are bound to see the candidate herself - repeatedly.

See election analysis, vote count updates, and video of Mayor Nutter's election views: http://go.philly.com/campaign2008