Hillary Clinton beat Democratic rival Barack Obama in Pennsylvania on Tuesday, prolonging an increasingly negative presidential nominating fight and keeping alive her slim White House hopes.

Mrs Clinton's 10-point win paid immediate dividends in fund-raising for the cash-strapped New York senator and shaved off some of Mr Obama's lead in popular votes and in delegates who select the Democratic nominee at the August convention.

Both candidates immediately looked to the next round of contests on May 6 in North Carolina, where Mr Obama is favoured, and Indiana, which is considered a toss-up.

"Some people counted me out and said to drop out, but the American people don't quit and they deserve a President who doesn't quit either," Mrs Clinton told cheering and chanting supporters in Philadelphia.

Mrs Clinton, whose campaign is in debt and running low on cash, urged backers to visit her Website and donate. A campaign spokesman said she raised $2.5 million in the hours immediately after she won Pennsylvania.

"The future of this campaign is in your hands," Mrs Clinton told her supporters. She was joined on stage by her husband, former President Bill Clinton, and daughter Chelsea.

Mrs Clinton survived a heavy advertising onslaught in Pennsylvania by Mr Obama, who outspent her by more than 2-to-1 in the first Democratic nominating contest in six weeks. She won with 55 per cent of the vote to Mr Obama's 45 per cent.

The contest in Pennsylvania, where 158 delegates were at stake, opened the final phase of the Democratic duel for the right to face Republican John McCain in November's presidential election. Nine more contests are scheduled before the voting ends on June 3.

Mr Obama, who narrowed a 20-point Clinton lead in opinion polls before falling short, already was looking ahead. He left Pennsylvania before the polls closed for an evening rally in Indiana.

"There were a lot of folks who didn't think we could make this a close race when it started," Mr Obama said in Evansville. "Six weeks later, we closed the gap. We rallied people of every age and race and background to our cause."

Turnout was heavy at many polling places, and a record number of Pennsylvanians had registered to vote.

The result followed Mrs Clinton's popular vote victories in Ohio, California, New Jersey and Texas, fuelling her argument she is the Democrat who can capture the big and diverse states where the party will need to do well in November.

Mr Obama, an Illinois senator who would be the first black US President, won nine of every 10 black voters and led among young and male voters. Mrs Clinton, who would be the first woman US President, won blue-collar workers, elderly voters and more than half of women.

She also won 58 per cent of those who decided in the last week, when Mr Obama was on the defensive in a debate over a series of campaign controversies and Mrs Clinton questioned his toughness in an ad featuring images of al Qaeda leader Osama bin Laden.

Voters were not happy with the race's negative tone. About two-thirds of Pennsylvania voters thought Mrs Clinton unfairly attacked Mr Obama, while about half thought Mr Obama had unfairly attacked Mrs Clinton, the polls showed.

Potential scenarios: Democrats seek endgame in White House race

The Democratic race between Senator Hillary Clinton of New York and Barack Obama of Illinois is likely to extend into June with neither candidate winning enough delegates to clinch the presidential nomination.

Mr Obama has a nearly insurmountable lead among pledged delegates who will help decide the candidate to face Republican John McCain in the November election.

But with nine nominating contests remaining until June 3, it is unlikely either candidate can reach the 2,024 needed to become the nominee. Following are some potential scenarios for settling the Democratic race:

• With Obama and Clinton stalled short of the magic number, the race could be decided by nearly 800 superdelegates - party officials and insiders who are free to back any candidate.

About 330 superdelegates remain uncommitted to either candidate, and if a big majority move to one candidate they could settle the race. The candidates have chased their support, but many superdelegates are postponing decisions until the nominating contests finish.

• Florida and Michigan were stripped of their delegates in a dispute with the national party, but held unsanctioned contests won by Mrs Clinton although no candidates campaigned in the states and Mr Obama was not on Michigan ballot.

Mrs Clinton has called for seating the delegates, which would probably still leave her trailing Mr Obama but would bolster her case to superdelegates that she is the candidate with the best chance to win in November.

Mr Obama says he will work with the Democratic National Committee to find a mutually agreeable solution, but opposes the seating of the delegates from the unsanctioned contests.

The national party says it will not change the rules at this late stage, and party officials in both states have ruled out re-votes that would allow delegates to be seated.

• With many of the superdelegates still undecided, the party could go to the nominating convention in Denver in August without a clear winner.

Party leaders, fearing the resulting chaos and damage to the party, have vowed to work to bring the race to a close before then. The prospect of a brokered convention, however, has led to speculation on a variety of scenarios.

Uncommitted Democrats like Al Gore and former candidates John Edwards and Joseph Biden could step in and convince either Mr Obama or Mr Clinton to step aside for the good of the party.

In a much more unlikely scenario, delegates could turn to a prominent Democrat like Mr Gore and ask him to be the nominee.