HAMMOND, Indiana (Reuters) - Democratic candidate Hillary Clinton on Friday vowed to stay in the White House race, rejecting a fellow senator’s suggestion that she pull out to improve their party’s chances in November.

“There are millions of reasons to continue this race,” New York Sen. Clinton told reporters after Vermont Democratic Sen. Patrick Leahy, a backer of Clinton rival Barack Obama, said it might be time for her to withdraw.

Many political experts believe Clinton will have a hard time defeating Illinois Sen. Obama for the Democratic nomination to run against Republican Sen. John McCain in November’s election.

Embarking on a six-day bus tour of Pennsylvania, which holds the next primary on April 22, Obama said the race for the Democratic nomination was “a good movie that lasted about a half an hour too long.”

Clinton pounced on the comment. “I like long movies,” she said.

Polls show Clinton leading by a wide margin in Pennsylvania but Obama picked up a valuable endorsement from popular Pennsylvania Democratic Sen. Robert Casey. The endorsement could help Obama with Catholics and blue-collar white voters who may otherwise be attracted to Clinton.

Casey said the campaign was a chance for change, new politics and healing. “I believe in my heart that there is one person who is uniquely qualified to lead us in that direction and that is Barack Obama,” Casey said in Pittsburgh as Obama stood on stage next to him.

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Many Democrats are worried that the increasingly bitter battle between Clinton and Obama can only help McCain who has clinched the Republican nomination. The Arizona senator on Friday aired the first TV ad for the November election.

Leahy said in a statement McCain was getting a free ride in the news media “because the Democratic candidates have to focus not on him but on each other.”

“Sen. Clinton has every right, but not a very good reason, to remain a candidate for as long as she wants to,” he said.

Clinton is behind Obama in the race for 2,024 nominating delegates but still thinks she can win and all Democrats should get a chance to vote.

CONTEST IS GOOD

“A spirited contest is good for the Democratic Party and will strengthen our eventual nominee,” she said.

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Democratic chairman Howard Dean appeared on morning news shows to warn that the Clinton-Obama battle risked demoralizing Democrats. He urged them both to tone down personal attacks but did not call on Clinton to withdraw.

Dean said he hoped the battle did not drag into the party’s nominating convention in late August in Denver, which Clinton has said was possible. He said he hoped the nomination would be wrapped up by July 1.

“The candidates have got to understand that they have an obligation to our country to unify. Somebody’s going to lose this race with 49.8 percent of the vote, and that person has got to pull their supporters in behind the nominee,” Dean told CBS’ “Early Show.”

Obama has a narrow lead in nominating delegates but is hundreds short of the 2,024 needed.

Dean said Democrats need to stay focused on taking the White House from Republicans in November.

“We do need to keep in mind that personal attacks now often do have the seeds of demoralization later on. So I want to make sure this campaign stays on the high ground,” he said.

McCain’s new ad was entitled “624787,” which was his Navy serial number. The video shows him repeating that number to an interviewer as he lay in a prison bed in Vietnam.

(Additional reporting by David Morgan, Matthew Bigg and Deborah Charles; writing by Steve Holland, editing by Alan Elsner)