Hillary Clinton and Barack Obama have waged a furious war of negative attacks, as tensions boil before their next crunch Democratic presidential primary in Pennsylvania.

Senator Clinton accused Mr Obama of cheerleadering for presumptive Republican nominee John McCain, after he said the Arizona senator would be a better president than George W Bush, apparently contradicting Democratic strategy.

Mr Obama's camp meanwhile looked past tomorrow's primary - which polls suggest Clinton will win, but short of the blowout her supporters hope for - to warn time was running out for the former first lady's uphill comeback bid.

Ms Clinton charged McCain would prolong the war in Iraq as well as Bush's "failed economic policies."

"We need a nominee who will take on John McCain, not cheer on John McCain, and I will be that nominee," Clinton said.

Mr Obama had earlier said in Reading, eastern Pennsylvania, that either Democrat would be better than McCain, and "all three of us would be better than George Bush."

Ms Clinton also laid into Mr Obama after he attacked her plan for universal healthcare, a key election issue in economically struggling Pennsylvania, which has seen its heavy manufacturing base shattered by foreign competition.

"This week we had a debate, and it showed you the choice you have," Ms Clinton told supporters in the eastern town of Bethlehem, which saw its 150-year-old steelworks go bankrupt a decade ago.

"It's no wonder that my opponent has been so negative these last few days of the campaign because I think you saw a real difference between us."

Mr Obama, leading Ms Clinton in nominating wins, the popular vote and elected delegates with only 10 contests to go, pressed home his own attacks, after accusing Ms Clinton of adopting "slash and burn" tactics on Saturday.

He said Ms Clinton was now using on him, the kind of withering attacks that she suffered as first lady in the White House between 1993 and 2001.

"I'm thinking well, you learned the wrong lessons from those Republicans who were going after you in the same ways using the same tactics all those years," Mr Obama was quoted as saying by CBS News.

Clinton 'running out of real estate'

Mr Obama’s ally, Richard Durbin meanwhile warned Ms Clinton was running out of "real estate" in her quest to deprive his fellow Illinois senator of the Democratic nomination.

"The math is very unforgiving at this point when it comes to delegate counts, and that's what it's all about," Durbin told Fox News yesterday.

"You understand the Clinton campaign is running out of real estate."

"Now, for Senator Clinton to have a viable chance of the nomination, she has to win 60 per cent of all the remaining contests," Durbin said.

Polls give Clinton Pennsylvania lead

A new MSNBC/McClatchy poll yesterday showed Clinton holding a five point, 48-43 per cent lead in Pennsylvania, consistent with previous surveys, pointing to a possible win, but short of the blowout many observers predict she needs.

"Would I like to win by double digits? Sure," said Ms Clinton backer and Pennsylvania Governor Ed Rendell on CBS.

"But I don't think that's going to happen," he said, adding that a four-to-seven point margin of victory would be "very significant."

Clinton insists on 'Obama's inexperience'

Ms Clinton hopes to win big to sow doubts about Mr Obama's viability in a general election against Republican presumptive nominee John McCain.

She argues that only she can capture big states like Pennsylvania and Ohio, packed with socially conservative blue-collar voters, that Democrats need to piece together a route back to the White House.

After a 15-month race, neither Democrat is expected to reach the tally of 2,025 nominating delegates to claim the nomination outright.

So Ms Clinton needs to convince nearly 800 superdelegates - top party officials who can vote how they like at August's party convention - that it would be too risky to pick the inexperienced Mr Obama to fight McCain in November.

McCain meanwhile heralded a possible general election attack on Mr Obama, rebuking the Democrat's links to a 1960s radical whom McCain called an "unrepentant terrorist."

The Arizona Senator said he was sure Mr Obama was "very patriotic" but said his relationship with William Ayers, a University of Illinois professor who was once part of the violent Weather Underground group, was "open to question."