JOHNSTOWN (AFP) -- Hillary Clinton and Barack Obama waged a furious war of negative attacks, as tensions boiled two days before their next crunch Democratic presidential primary in Pennsylvania.

Clinton accused 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.
Obama's camp meanwhile looked past Tuesday'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.
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.
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.""
Clinton also laid into 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,"" 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.""
Obama, leading Clinton in nominating wins, the popular vote and elected delegates with only 10 contests to go, pressed home his own attacks, after accusing Clinton of adopting ""slash and burn"" tactics on Saturday.
He said 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,"" Obama was quoted as saying by CBS News.
Obama ally, Richard Durbin meanwhile warned 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 Sunday.
""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 percent of all the remaining contests,"" Durbin said.
A new MSNBC/McClatchy poll Sunday showed Clinton holding a five point, 48-43 percent 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 Clinton backer and Pennsylvania Governor Ed Rendell on CBS.