Senator Clinton’s victory in a hard-fought Pennsylvania primary keeps her presidential campaign alive as she looks for an infusion of cash and the backing of superdelegates to carry her to the next contests in Indiana and North Carolina.

With 88% of the precincts reporting, Mrs. Clinton was leading Senator Obama, 55% to 45%, in an election that saw heavy turnout among Democrats across the Keystone State. The television networks and the Associated Press called the race for Mrs. Clinton about an hour after the polls closed at 8 p.m.

Appearing in Philadelphia last night, a triumphant Mrs. Clinton told a raucous crowd of supporters that “the tide is turning” in the race for the Democratic presidential nomination.

“Some people counted me out and said to drop out,” she said as her supporters erupted in boos at the suggestion, “but the American people don’t quit and they deserve a president who doesn’t quit either.”

Mrs. Clinton made a direct appeal for financial contributions, telling supporters that she “can only keep winning if we can keep competing with an opponent who outspends us so massively.”

Mr. Obama spoke to a rally last night in Indiana, where he first congratulated Mrs. Clinton before laying out an implicit but extensive argument against her candidacy. He criticized the presumptive Republican nominee, Senator McCain, as offering a continuation of President Bush’s administration, but he questioned whether Mrs. Clinton would present a clear contrast. “We already know that John McCain offers more of the same,” Mr. Obama said. “The question is not whether the other party will bring about change in Washington — the question is, will we?”

He added: “We can be a party that says and does whatever it takes to win the next election. We can calculate and poll-test our positions and tell everyone exactly what they want to hear. Or we can be the party that doesn’t just focus on how to win but why we should.”

The win for Mrs. Clinton, which was widely expected after several polls showed her leading, may quiet calls for her immediate exit from the Democratic race, but it will not likely make her path to the nomination much easier.

The result is unlikely to significantly erode Mr. Obama’s delegate lead, and Mrs. Clinton’s attempts to raise doubts about the Illinois senator’s electability have thus far drawn few undecided superdelegates to her corner.

The New York senator is hoping that her Pennsylvania win will boost her debt-ridden campaign and give her more time to make her case to the roughly 300 undecided superdelegates who will, in large measure, decide the Democratic nomination. Amid reports that Mr. Obama might announce several superdelegate endorsements after yesterday’s vote to blunt any momentum for Mrs. Clinton, a senior Democrat in the Clinton camp told The New York Sun yesterday she would soon roll out her own group of new superdelegates, including several from Pennsylvania.

The chairman of the Democratic National Committee, Howard Dean, has urged superdelegates to publicly announce their decisions soon to prevent the primary fight from carrying into the party’s nominating convention late this summer.

With Mr. Obama expected to retain his advantage in pledged delegates through the final nine contests, Mrs. Clinton will have to persuade superdelegates to overturn that edge and risk alienating a substantial portion of the Democratic electorate, and particularly African-Americans, who may question the legitimacy of her nomination.

By all accounts, Pennsylvania had set up well for Mrs. Clinton, with its large pockets of white, blue-collar voters who have formed her base throughout the campaign. The primary was the first Democratic primary in seven weeks, and Mrs. Clinton began the race with a lead of more than 15 points in many polls. She relied heavily on her family’s connections to the state, often reminding voters that her father grew up in Scranton.

After initially downplaying the state, Mr. Obama made an aggressive push of his own in the last few weeks, using his fund-raising advantage to outspend Mrs. Clinton by a 3-to-1 margin.

Exit polls showed that while Mr. Obama carried younger voters, Mrs. Clinton won the overwhelming support of seniors, 60% to 39%. Mr. Obama won with voters who only recently registered as Democrats in Pennsylvania. Mrs. Clinton, meanwhile, carried 58% of the voters who said they decided in the last three days, suggesting that Mr. Obama’s shaky performance in last week’s debate and late ads by the Clinton campaign may have hurt him, the polls showed.

The two campaigns worked furiously through the final days and even hours to set expectations for the results, with the Obama campaign arguing that it would take a Clinton rout to change the dynamics of the race. “Behind in delegates and sporting a 14-30 primary record (not good enough even to make the playoffs in the NBA Eastern Conference), the Clinton campaign needs a blowout victory in Pennsylvania to get any closer to winning the nomination,” the Obama camp wrote yesterday in a memo to reporters.

The candidate himself seemed to depart from that line later in the day in an interview with XM Satellite Radio. “A win is 50 plus one,” Mr. Obama said. “So if Senator Clinton gets over 50%, she’s won the state and I don’t try and pretend that I enjoy getting only 45% and that’s a moral victory. You’ve lost the state.”

Mrs. Clinton suggested the pressure should be on Mr. Obama to win a large state key to Democratic hopes for victory in November. “I don’t think the margin matters. I think a win is a win,” the former first lady said in an appearance on NBC’s “Today” show yesterday morning.

She repeated the refrain while greeting voters later in a suburb of Philadelphia. “Why can’t he close the deal?” she asked.

Although Mr. Obama retains a lead in delegates, the popular vote, and states won, the Clinton campaign has consistently pointed to his failure to capture the largest states as presenting a liability heading into the general election.

In a show of confidence that the race would continue, Mrs. Clinton scheduled events in Indiana for today and later this week, and her increasingly cash-strapped campaign launched new television ads in Indiana and North Carolina, which hold primaries May 6, even before the polls closed in Pennsylvania. On Monday night, staffers at her headquarters in Northern Virginia were coordinating plans for Montana, which votes on June 3.

The increasingly negative tone of the race has concerned some party leaders, including supporters of both candidates who fear the impact of a nasty, protracted fight on the eventual nominee’s chances of defeating the presumptive Republican nominee, Senator McCain, in November.

“I don’t like negative campaigns between Democrats,” a staunch Clinton supporter, Rep. Carolyn Maloney of New York, said in an interview yesterday. She added that she thought neither Mrs. Clinton nor Mr. Obama had become too negative.

Mrs. Maloney, who campaigned for Mrs. Clinton in Pennsylvania over the weekend, dismissed the notion that a narrow victory could force her from the race. “She has announced her campaign is going forward, so end of story. She is going forward,” Mrs. Maloney said. Of Mrs. Clinton’s chances to capture the nomination, she said: “It’s not over ’til it’s over.”

Another New York supporter of Mrs. Clinton, Rep. Joseph Crowley of Queens and the Bronx, voiced even fewer worries that the back-and-forth between the two candidates had damaged the party’s standing for the fall. “The reality is this is a tough game,” Mr. Crowley said. “We want whoever our standard-bearer is to be battle ready.”