Hillary Clinton and Barack Obama accused each another of stooping to negative scare tactics, as tensions boiled two days before their next crunch Democratic presidential primary in Pennsylvania.

Obama's camp meanwhile looked past Tuesday's election -- 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 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 attacks, after accusing Clinton of adopting "slash and burn" tactics on Saturday.

On his own tour across the gritty northeastern state, 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 in the eastern city of Reading.

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 per cent 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.

"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 hopes to win big to sow doubts about 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 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 Obama to fight McCain in November.

As an advertising war raged over Pennsylvania airwaves, Obama stopped by a diner in the eastern town of Robesonia, and chatted with customers about high gasoline prices.

Not to be outdone, Clinton toured a family restaurant in Abington, outside Philadelphia, and spoke in depth to voters about their problems, and cradled a young child, like Obama seeking a time on local news broadcasts.

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

McCain said he was sure 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."