PITTSBURGH — Barack Obama got a surprise boost in the last big state of the long Democratic campaign Friday with an endorsement from Pennsylvania Sen. Bob Casey Jr., while another Obama supporter sought to nudge Hillary Rodham Clinton out of the race.

Clinton leads by double-digits in Pennsylvania polls, and Obama hopes Casey's endorsement will earn him a second look from the state's white, working class and Catholic voters _ groups that have leaned toward Clinton in other Democratic contests this year.

Clinton, on the other hand, is hoping a victory in Pennsylvania will help persuade party "superdelegates" to support her and allow her to catch Obama in the race for National Convention delegates.

Sen. Patrick Leahy, a Vermont senator who endorsed Obama in January, said she was never going to win enough delegates, and he suggested she should throw in the towel in "the interests of a Democratic victory in November." A number of Democrats have expressed concern that Republican John McCain is getting a head start while Obama and Clinton fight on.

Undeterred, Clinton said the competition would only strengthen the party in the long run.

"This spirited, exciting contest is actually a real plus for us," she said while campaigning in Indiana, which has its primary two weeks after Pennsylvania's April 22 vote.

McCain launched his first television ad of the general election campaign Friday, portraying himself as a courageous leader with the knowledge and experience to keep the country safe as a wartime commander in chief. "The American president Americans have been waiting for," the ad says, juxtaposing footage of the Arizona senator with clips of him as a prisoner of war in Vietnam three decades ago.

Casey, the son of a popular late governor, had said earlier this month he would not endorse before the Pennsylvania primary out of concern for party unity. But he joined Obama at a boisterous rally kicking off a six-day bus trip through the state, where current Gov. Ed Rendell has been campaigning hard for Clinton.

Coming so late in the campaign season, Pennsylvania will play an unexpectedly key role this year. The state's primary will allocate 158 delegates, the biggest prize left in the drawn-out nomination battle.

After the Pittsburgh rally, Casey said of Obama: "I believe in this guy like I've never believed in a candidate in my life, except my father."

Unfazed, Clinton noted her own roster of high-powered endorsements including Philadelphia Mayor Michael Nutter and anti-war Rep. John Murtha in addition to Rendell.

The former first lady's campaign used the criticism of her candidacy as a fundraising tool.

"Have you noticed the pattern?" Clinton wrote in an e-mail to supporters. "Every time our campaign demonstrates its strength and resilience, people start to suggest we should end our pursuit of the Democratic nomination. Those anxious to force us to the sidelines aren't doing it because they think we're going to lose the upcoming primaries. The fact is, they're reading the same polls we are, and they know we are in a position to win."

Obama is hoping for a showing in the state that would force her to rethink whether she will push ahead. Obama holds the lead in pledged delegates, and Clinton would need landslide wins in the remaining 10 contests to overtake him in that category, an unlikely scenario.

The Casey endorsement also was a welcome change of subject for Obama after two weeks of grappling with the uproar over incendiary comments from his pastor, the Rev. Jeremiah Wright. Recent national polls indicate that the Illinois senator, bidding to become the first black president, handled the situation well with a major speech on race.

Casey is a first-term senator, a Catholic who, like his father, is known for his opposition to abortion and his support of gun rights.

"I really believe that in a time of danger around the world and in division here at home, Barack Obama can lead us, he can heal us, he can help rebuild America," Casey told the crowd in Pittsburgh.

His move on Friday could be seen in part as a political jab at the Clintons. Bill Clinton was the Democrats' presidential nominee in 1992 when Casey's father was not given a prime-time speaking position at the party's convention, which outraged many of the state's conservative Democrats.

Casey said enthusiasm for Obama by family members _ including his four daughters _ was part of his reason for endorsing now. He said he did a lot of thinking over the Easter holiday.

"When people you love and people you respect in your family or community are not just supportive of him, but incredibly enthusiastic about his candidacy, that has had an impact on my thinking and the way I approach this," he said. "I didn't think that at this point in our history with the stakes so high that I could stay on the sidelines once I had made a decision about who, the person I'd vote for."

He said the decision had nothing to do with the Casey history with the Clintons.

Obama himself challenged Clinton's argument that her experience in government would make her a better candidate in November against McCain.

"If the contest between McCain and the Democratic nominee is who's been there longer, John McCain wins!" he said to laughter from the crowd.

"If the argument is who is going to pursue a foreign policy like George Bush's, then John McCain wins. If that's the criteria for being tough, if that's the criteria for answering the 3 o'clock phone call after you voted for the war in Iraq and you went along with George Bush's policies when it came to Iran and not talking to leaders that we don't like, then John McCain wins that fight."

Leahy told Vermont Public Radio in an interview that aired Thursday: "There is no way that Sen. Clinton is going to win enough delegates to get the nomination."

In a statement issued Friday, he said Casey's endorsement was the latest sign.

"Senator Clinton has every right, but not a very good reason, to remain a candidate for as long as she wants to. As far as the delegate count and the interests of a Democratic victory in November go, there is not a very good reason for drawing this out. But as I have said before, that is a decision that only she can make," Leahy said.

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Associated Press Writer Beth Fouhy in Mishawaka and Kimberly Hefling in Washington contributed to this report.