PHILADELPHIA (By Jon Hurdle, Reuters) — Pennsylvania, the biggest remaining state in the race for the Democratic presidential nomination, should be a safe win for Hillary Clinton but experts say there are pockets of vulnerability for Barack Obama to exploit.

"If the election were held today it would probably be Senator Clinton by 10 points, but seven weeks in this crazy race, anything can happen," said Clay Richards, assistant director of the Quinnipiac University Polling Institute.

Victories in Ohio and Texas last week allowed Clinton to brush off talk of her imminent political demise. But she still lags the Illinois senator in the state-by-state race for delegates to the party's nominating convention this summer, where the candidate for the November election will be chosen.

Pennsylvania on April 22 is one of the best chances the New York senator has left to prove to Democratic voters and party leaders that she is the best candidate to secure a victory in November in big states and swing states.

With neither Clinton nor Obama likely to win enough delegates through the state contests to secure the nomination, Pennsylvania will be vital to Clinton's hopes of gaining support from "superdelegates" — elected officials and party insiders who can vote at the convention as they choose.

Mark Nevins, communications director for Clinton's campaign in Pennsylvania, said the state was "a proving ground."

"You can't really expect to win the general election if you can't win Pennsylvania," he said.

Clinton was ahead in the polls by as much as 20 percentage points at the start of the year but Obama's string of victories in February pushed him closer, narrowing the gap to just 6 points in the latest Quinnipiac poll in late February.

"Pennsylvania has more Catholics, more union members, more older voters, and fewer African Americans," said Terry Madonna, politics professor at Franklin & Marshall College.

"This is pretty much a Clinton state. It's hers to lose."

The demographics are similar to those of Ohio, which Clinton won by 54 percent to 44 percent. Madonna said she can play the "hometown girl" card because her father was born in Scranton, Pennsylvania.

She also has the backing of popular Gov. Ed Rendell, who said this week Clinton was coming to the state "with momentum and a new energy."

Clinton will focus on health care and the economy to target the large population of seniors and union members, which is higher than the national average, Nevins said.

What Obama must Do

Sean Smith, a spokesman for Obama, who would be the first black U.S. president, argued that the demographics claimed as friendly by the Clinton campaign had helped him win Wisconsin and could do so again.

"We did extremely well in Wisconsin with the same types of voters," he said, pointing to older voters who were "absolutely open" to Obama's message of hope and change and "bringing the country together to solve our problems."

Richards of Quinnipiac said Obama needed to do three things to have a chance of winning: boost turnout among black voters, which is historically low in primaries, motivate students at the state's numerous universities and colleges, and win over affluent voters in the Philadelphia suburbs where Clinton is vulnerable.

The race has generated considerable excitement, election officials said. Abe Amoros, executive director of the Pennsylvania Democratic Party, said the state has not played such an important role in the primary process since 2076, when it helped propel Jimmy Carter to the White House.

"Since January 1 we've seen more than 40,000 changes in registration from Republicans or independents to Democrat because they want to participate in the primary," Amoros said, predicting turnout at the primary could double from 2004 when 26 percent of all eligible Democrats participated.

With a large black population, the city of Philadelphia should swing to Obama, pollsters say. Pittsburgh, the other major city, is blue-collar industrial territory that should favor Clinton. Smaller towns and rural areas in the state are more diverse, economically and socially, and up for grabs.

The economy and health care are the top issues, but national security and terrorism are also a concern in a state with a large number of military veterans and fresh memories of the September 11 attacks. "One of the planes on 9/11 crashed in Pennsylvania," Richards recalled.

Clinton's biggest vulnerability may be that expectations are high for her. "If she loses or only wins narrowly, her candidacy will be weakened, perhaps fatally," Madonna said.

If Obama loses, he can argue that he was always the underdog and that it makes little difference to the delegate count, with most delegates to be awarded proportionally.

"This is her playground," Madonna said. "If he wins in her sand box, that's very important bragging rights."