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 to stay in the race. Obama hopes to deliver a knock-out blow by outspending her in advertising.

With tens of millions from fundraising at his disposal, he plans to spend well over $2 million on 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 $91,000 worth.

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. The latest poll, by Zogby, puts Clinton on 47 per cent to Obama’s 43 per cent. While the polls throughout the primaries have been notoriously unreliable, the pattern of the polls over the past few weeks have consistently shown Clinton's once formidable double-digit lead is shrinking.

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

He told CNN on Thursday 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.