Sen. Bob Casey's decision to embrace the candidacy of Sen. Barack Obama yesterday was aimed at two very different audiences: the Pennsylvania Democrats who backed him in a landslide victory over former Sen. Rick Santorum two years ago and the party's scores of unpledged superdelegates nationwide, who may be called on to settle the nomination fight.

Democrats have had registration leads in Westmoreland County -- where the two senators appeared together last night after the surprise morning announcement -- and nearby rural counties for decades.

But in recent years, voting results have belied the registration numbers and have gone Republican. President Bush carried Westmoreland County twice. But unlike other Democrats, Mr. Casey, with an economically liberal but socially conservative profile, has been able to count on their support.

Whether he can translate that popularity to votes for Mr. Obama won't be known for three-and-a-half weeks.

This strain of traditional Democrats is also a bulwark of Sen. Hillary Rodham Clinton's hopes for a game-changing victory in the state. Winning Ohio on March 3, she was able to pile up big leads among working class white voters and Catholics, constituencies that are traditional strengths for Mr. Casey.

Superdelegates and senior party officials, the other targets of Mr. Casey's endorsement, are coming under increasing pressure to serve as referees for a contest that will not be resolved by the votes of pledged delegates yet to be elected in Pennsylvania and the handful of other states still remaining on the long Democratic nomination road.

"Children have been born and are walking and talking since I began running for president," Mr. Obama said last night.

Former Vermont Gov. Howard Dean, the Democratic National Committee chairman, said yesterday that he expected the superdelegates to decide on one candidate or another by July 1.

Sen. Patrick Leahy, D-Vt., an Obama supporter, issued a statement yesterday saying that, "Sen. Casey's endorsement of Sen. Obama in Pennsylvania is the latest sign of how the race is going" and, in a public radio interview, suggested Mrs. Clinton pull out.

Speaking to reporters yesterday, Mr. Casey said he made his decision because, "I believe in this guy like I've never believed in a candidate in my life, except my father."

Asked about the calls from other Obama backers for Mrs. Clinton to drop from the race, Mr. Casey demurred.

"I'm concentrating on one thing right now, the April 22 Pennsylvania primary," Mr. Casey said. "We'll see where we are at the end of April."

Mr. Obama opened his campaign yesterday with an appearance at Soldiers & Sailors Military Museum and Memorial in Oakland. Mrs. Clinton had appeared in the same hall two weeks ago, showcasing her own Pennsylvania endorsements, including Gov. Ed Rendell, Allegheny County Chief Executive Dan Onorato and Pittsburgh Mayor Luke Ravenstahl.

Mark Nevins, a spokesman for the Clinton campaign, pointed to her early support in dismissing the impact of the Casey decision.

"At the end of the day, there is a clear choice in this election between action and words," Mr. Nevins said. "[Between] experience and rhetoric, and we trust the people of Pennsylvania to make the right decision."

In reaction to Mr. Casey's decision and Mr. Leahy's statement, Mrs. Clinton told the Associated Press last night: "There are millions of reasons to continue this race: People in Pennsylvania, Indiana and North Carolina -- and all of the contests yet to come. This is a very close race, and clearly I believe strongly that everyone should have their voices heard and their votes counted."

Until yesterday, Mr. Obama's highest profile allies in the state were two Philadelphia area congressmen, Reps. Chaka Fattah and Patrick Murphy.

Overall, Mrs. Clinton retains the superdelegate lead, but her edge has eroded as Mr. Obama has garnered the majority of senior party officials declaring in the past two months.

Mr. Obama's newest high profile supporter told the crowd at Soldiers & Sailors that he came to his decision because, "This campaign is a chance for America to chart a new course, to go down a different path, a path, first of all, of change; a path of a new kind of politics and finally a path of hope and healing ... I believe in my heart that there's one person who uniquely qualified to lead us in that new direction and that's Barack Obama."

As Mr. Casey spoke, the words from the stage were overwhelmed by chants of "Yes we can; yes we can," from the crowd.

"You know, I did not press him on this endorsement. You know there were some people that I was nagging all the time," Mr. Obama said. "I told him I'd love to have his support, but I understood that, you know, we're behind in the Pennsylvania polls. I just want to say it would have been easy for Bob just to stay out of it, just to stay neutral. I think everybody would have accepted that.

"But when he called me and said, 'I think this is the right thing to do,' it meant as much to me as any endorsement that I've received on the campaign trail," Mr. Obama added.

The superdelegate's embrace came as the Democratic presidential contender embarked on a six-day bus tour of Pennsylvania. After leaving Oakland, he stopped at U.S. Steel's Edgar Thomson Works in Braddock and met with a group of workers.

The decision by Mr. Casey, the top vote-getter in Pennsylvania history, was unexpected as he had said repeatedly that he planned to remain neutral in the presidential contest at least until after the end of the state's April 22 primary. Mr. Casey had said it was important for some Democratic elders to remain neutral to put the frayed edges of the party together after the bruising nomination battle.

Mr. Casey said he had notified Mrs. Clinton of the planned endorsement the previous evening.

"She was very gracious. She is a great senator and a great leader," Mr. Casey said.

Mr. Casey said one factor in his decision was his admiration for the speech on race and politics that Mr. Obama delivered last week in Philadelphia.

"I thought it was a magnificent speech," Mr. Casey said. "And I think that speech alone tells a lot about who he is, taking on tough issue, how honest he's been ... and also his way of not adding heat to the fire, when things are hot, to be able to exercise the kind of calm, steady leadership that you'd want."

At Soldiers & Sailors yesterday, he said his decision would be applauded by yet another audience -- his four daughters, all of whom, he recounted, were Obama supporters before their dad joined them.