WASHINGTON - Hillary Clinton blew past Barack Obama to take the Pennsylvania primary Tuesday after a fierce battle to remain in the roller-coaster Democratic presidential nomination race.

It was a crucial victory for Clinton, since a loss would have crushed her cash-strapped campaign for the White House. But she still faces major challenges amid growing frustration among some Democratic party officials who'd like to see her drop out.

The Pennsylvania win, on the strength of votes from women, white men and blue-collar workers, keeps Clinton in the game at least until the next primaries in Indiana and North Carolina in two weeks.

Clinton was winning 55 per cent of the vote to 45 per cent for Obama with 99 per cent of the vote counted.

She won at least 66 delegates to the party's national convention, with 35 still to be awarded. Obama won at least 57. A final count could come Wednesday, or later.

She was hoping for a resounding, double-digit win to bolster her argument that Obama may be the front-runner but he can't deliver crucial states in this fall's election campaign against Republican John McCain.

Clinton badly needs to convince superdelegates - legislators and party insiders who will likely end up deciding the race - that she's best poised to beat McCain.

"It's a long road to 1600 Pennsylvania Avenue and it runs right through the heart of Pennsylvania," she told cheering supporters in Philadelphia.

"You listened and today you chose," said Clinton, who appealed for much-needed funds to sustain her uphill fight.

"Because of you the tide is turning."

Obama, who's been outspending Clinton by more than two to one during an intense, bitter six-week campaign, had predicted she'd beat him in Pennsylvania, but not by much.

After nearly four months of voting, Obama is still beating her in the number of states won, awarded delegates for the party convention in August and the popular vote.

Speaking to supporters in Evansville, Indiana, Obama congratulated Clinton, saying: "She ran a terrific race."

But he went on to criticize the "bickering" and "tit-for-tat" that's characterized an increasingly personal race and took a swipe at Clinton without naming her for using fear tactics.

Her final television ad in Pennsylvania included an image of terrorist Osama bin Laden and suggested Obama isn't up to the challenges posed by the presidency.

"That kind of politics is not why we're here tonight," he said. "We are not as divided as our politics suggests."

Clinton was winning among women, who were casting about 60 per cent of the votes in Pennsylvania, and captured many of the three in 10 voters aged 65 or over.

She was also dominating among blue-collar voters and white men, while Obama was favoured by blacks, the affluent and about one in 10 voters who recently switched to the Democratic party.

The economy was top of mind for voters. More than 80 per cent said they thought the country is already in a recession.

There's enormous pressure on Clinton to perform in the nine remaining contests ending June 3 if she's to retain a serious shot at the Oval Office.

Obama is already favoured in North Carolina, where there's a large black population.

He also raised twice as much money as Clinton in March, with $42 million in the bank. She pulled in $20 million but is still facing a $10.3-million debt.

Despite his momentum, Clinton, who has taken large states like California, New Jersey, Ohio and Texas since voting began in January, has refused to give up.

Since neither candidate will get the required number of pledged delegates needed - 2,025 - to win the Democratic nomination during regular voting, the role of superdelegates has become paramount.

Obama was leading Clinton with 1,648.5 to 1,509.5 in overall delegates going into Pennsylvania, where there were 158 up for grabs. Some four million Democrats were registered to vote.

Both candidates tried to appeal to anti-free trade voters in the state by promising to renegotiate the North American trade pact to include protections for workers and the environment.

Clinton likely benefited from the fact that Pennsylvania conducts "closed" primaries. Independents who've favoured Obama in the past weren't allowed to vote.

Obama was running ahead in Philadelphia, which has a large African-American population, and surrounding suburban counties full of upscale voters.

Clinton was favoured in the more blue-collar city of Pittsburgh in western Pennsylvania and dominated rural counties and gun owners.

The New York senator and former first lady has been down before, only to spring back and confound the pundits.

Obama knocked her off stride with an impressive win in the first vote in Iowa in January and his message of hope and bridging the partisan divide caught fire among voters.

But she surged back in the second contest in New Hampshire, fought to a draw on Super Tuesday in February and then won Ohio on March 4 to revive her candidacy.

Both candidates pulled out all the stops in Pennsylvania but were beset by blunders.

Obama has fought charges of elitism after suggesting that small-town Americans "cling to guns or religion or antipathy to people who aren't like them" because they're "bitter" about the dismal economy.

He's also faced questions about his ties to contentious former pastor Rev. Jeremiah Wright and 1960s radical William Ayers.

Clinton, meantime, was forced to concede that she hadn't landed under sniper fire in Bosnia when she was first lady, even though she said several times that she had.

And she replaced her chief strategist, Mark Penn, after he met with officials of the Colombian government seeking passage of a free trade agreement that she opposes.

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