Barack Obama is to mount the biggest advertising blitz of the presidential campaign this weekend ahead of Tuesday's Pennsylvania primary in an attempt to force Hillary Clinton out of the race.

With pressure mounting among senior Democratic figures to bring the contest to an early close, Clinton needs a large margin of victory in Tuesday's primary just to stay in the race. Obama is hoping to deliver a knock-out blow by outspending her in advertising.

Obama, with tens of millions at his disposal from fundraising, is planning to spend well over $2m in ads in the run-up to the primary, at least twice as much as the cash-strapped Clinton campaign.

In Philadelphia, which could determine the outcome, he reserved for the final days $465,000 worth of television and radio spots while Clinton reserved only $91,000.

Obama's strategy – which he has deployed time and again during his political career – is that the more money spent on advertising, the higher the odds of winning an election.

Clinton, who, like Obama, is criss-crossing the state tomorrow, Sunday and Monday, needs to beat Obama by more than 10% of the vote to head off calls from senior Democrats to withdraw.

The latest poll, by Zogby, puts Clinton on 47% to Obama's 43%. While the polls throughout the primary season have been notoriously unreliable, the pattern of the polls over the last few weeks have consistently shown a shrinking of Clinton's once formidable double-digit lead.

The Democratic chairman, Howard Dean, reflecting concern within the party that the bickering between Clinton and Obama could damage the party's chances in the November general election, signalled again that he would like to see an early end to the contest.

He told CNN yesterday he needed the superdelegates - members of Congress and other senior Democrats who hold the balance of power - to declare as a matter of urgency which of the two they support.

"I need them to say who they're for starting now," he said. "We cannot give up two or three months of active campaigning and healing time. We've got to know who our nominee is."

Obama is so far ahead that it is near impossible for Clinton to catch him in the remaining eight contests after Pennsylvania, beginning with North Carolina and Indiana on May 6 and ending with Montana and South Dakota on June 3.

Obama leads Clinton in the overall delegate count, 1,645 to 1,504. Clinton has a slight edge in superdelegates, with 248 backing her compared with 226 for Obama, but since March 3, he has gained 80 and she has added only five.

Obama has lost some momentum in recent weeks, forced on the defensive by an aggressive Clinton team and his dismissive 'bitter' comment about small-town residents that has hurt his campaign.

He emerged battered from a debate with her in Philadelphia on Wednesday night and today hinted he may not, after all, take part in the next debate with her, planned for Raleigh, North Carolina, on April 27.

"I'll be honest with you, we've now had 21(debates)," he said. "It's not as if we don't know how to do these things. I could deliver Senator Clinton's lines; she could, I'm sure, deliver mine."

Clinton had formally agreed to participate in the North Carolina debate but Obama has still to accept.

Obama complained that the first 45 minutes of the Philadelphia debate was devoted to negative questions and comments directed against him from Clinton and the television moderators.

Clinton, interviewed by a Fox television affiliate in Philadelphia today, accused him of running away. "We need a president who is going to be up there fighting everyday for the American people and not complain about how much pressure there is, and how hard the questions are," she said.