Landslide victories over the weekend have given Barack Obama an opportunity to break the deadlock with Hillary Clinton as the two Democratic presidential candidates hurtle into the “Potomac primary” tomorrow.

Although Mr Obama’s wins – sweeping through Washington State, Nebraska, Louisiana and the US Virgin Islands on Saturday and taking yet another caucus victory in Maine last night – still leave him narrowly behind in the race for delegates, the next set of elections may result in his inching ahead for the first time. Mr Obama led Mrs Clinton by 58 per cent to 41 per cent in Maine with 70 per cent of results reported.

Though she denied that flagging results were to blame, Mrs Clinton last night announced the replacement of Patti Solis Doyle, her campaign manager, with Maggie Williams, a key aide during the tenure of her husband, Bill,in the White House.

Mr Obama is increasingly emphasising his appeal as a general election candidate who can pick up independent and even Republican votes against John McCain in every corner of the nation. “Today, voters from the West Coast to the Gulf Coast to the heart of America stood up to say ‘Yes, we can’ . . . we won north, we won south and we won in between,” he told 5,000 Democrats at a party dinner on Saturday night in Richmond, Virginia.

Mrs Clinton had earlier addressed the same event without referring to the results. Her aides have sought to play down the significance of the latest results, pointing out that Mr Obama had been “saturating the airwaves with 30 and 60-second ads” in states that he had been expected to win. They added that other looming contests this month, including tomorrow’s, were also “more favourable to the Obama campaign”.

In Louisiana on Saturday Mr Obama received huge support from the black vote, which is expected to be influential again tomorrow, when Virginia, Washington DC and Maryland vote. A poll in Virginia yesterday showed Mr Obama on course to win by 53 per cent to 37 per cent.

Both candidates campaigned in the state yesterday but Mrs Clinton is already looking ahead to March 4, when Texas and Ohio hold the type of big primaries where she performs well.

Her narrowing lead in delegates – 1,095 to 1,070, according to the Associated Press – is now sustained only by so-called super-delegates comprised of Democratic Party leaders and members of Congress who account for about a fifth of the 2,025 needed to win the nomination. More than half remain uncommitted and frantic efforts are under way by both campaigns to win over a group that may hold the balance of power at the Democratic convention in August.

Mrs Clinton and her husband are calling in favours dating back to their time in the White House. Mr Obama’s campaign team is putting intense pressure on super-delegates from states where he has won not to frustrate the will of voters.

In the Republican contest Mike Huckabee stung John McCain, the front-runner, by trouncing him in the Bible Belt state of Kansas on Saturday. He also narrowly won the Louisiana primary and ran close in Washington state. The Baptist preacher yesterday continued to resist calls to quit the race given that Mr McCain has taken a near-insurmountable lead. “I didn’t major in math. I majored in miracles, and I still believe in them,” he said.

In a TV interview yesterday President Bush acknowledged that Mr McCain “has some convincing to do” before he was accepted as a “solid conservative” but added: “I will be glad to help him if he is the nominee.”

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