Even as Democrats fret over the damage that the battle for the party’s presidential nomination has already done to their chances of regaining the White House, Hillary Clinton’s campaign is talking of a “nuclear option”.

If she wins in Indiana and does well enough in North Carolina in today’s crucial primaries, her campaign believes it may yet be able to overturn party rules that currently ban delegates awarded in her vexed “victories” in Florida and Michigan.

The theory is that it would help her to overtake Barack Obama in the popular vote, cut his lead among elected delegates to less than 100, and give wavering super-delegates an excuse to choose her as the presidential candidate in defiance of the results from Democratic primaries and caucuses.

There would, of course, be a high price to pay, with Mr Obama certain to challenge – all the way to the convention itself – any such effort to move the goalposts. One of his supporters, the former Senator Tom Daschle, gave warning yesterday that this would be a “disaster” and even a Democrat backing Mrs Clinton suggested privately that many would think twice about provoking a “fist fight over credentials” for the convention.

Although it is possible that Mr Obama could yet deliver a knock-out blow to Mrs Clinton tonight by winning a landslide in North Carolina and securing victory by any margin in Indiana, neither side really expects this. Mr Obama wearily told morning TV shows that the party would probably wait until after the last primaries on June 3 before making “a decision [on] who the Democratic nominee is going to be”. His campaign issued a memo emphasising the relentless logic of “the maths that shows he is now only 276 delegates away from securing the Democratic nomination”.

Mrs Clinton claimed that after starting far behind Mr Obama in North Carolina and Indiana she had “closed the gap”, while her campaign sought to play down expectations about tonight’s results. Geoff Garin, chief strategist, said the campaign team “should be judged by the progress we have made in the past couple of weeks . . . we have got things moving in the right direction”.

But James Carville, a former adviser to Bill Clinton and a prominent supporter of her campaign, suggested that she had to do better than that, saying: “The onus is on her. If she wins Indiana and North Carolina, she’s the nominee. She’s got to shock the system and she may be shocking it.” Privately, even aides acknowledge that Mrs Clinton cannot make up all the ground that she lost against Mr Obama in the hunt for delegates unless votes from Florida and, perhaps, Michigan are counted.

The Democratic party leadership is refusing to seat the 366 delegates from Florida and Michigan because the two states broke rules by staging primaries too early. If Mrs Clinton can get some or all of them reinstated, she would not only hack a big slice off Mr Obama’s delegate lead but also catch him in the overall vote.

Mr Obama, whose name was not even on the ballot in Michigan and who did not campaign in Florida, has said that the only fair solution is to award delegates from these two states on a 50/50 basis. Mrs Clinton says that this would ignore the wishes of 2.5 million voters who took part in the primaries and “expect to have their voices heard”.

She will reprise the events of 2000 when Al Gore beat George Bush in the popular vote in the presidential election only to lose out among elected delegates. And she needs little invitation to remind the party of the lessons from that bitterly-contested campaign about the importance of winning states such as Florida, which ultimately handed the White House to Mr Bush.

The “nuclear option” could be deployed on May 31 at a meeting of the Democrats’ Rules and Bylaws Committee, where some reports suggest that Mrs Clinton has a built-in majority. But even if this is the case – and there are many who suggest it is not – there would still be hurdles for her to overcome. Mr Obama’s campaign would do its best to overturn the decision by appealing to a perhaps more sympathetic Convention Credentials Committee in August.

Howard Wolfson, Mrs Clinton’s spokesman, swatted away questions on the subject. “Anyone who can tell you that they know how this race plays out between now and the convention is likely to get it wrong,” he said. “We have seen constantly there have been twists and turns in the road and there are important contests to come between now and June 3.”

Indeed, Mrs Clinton’s best chance of getting any of the Florida or Michigan delegates seated is to keep winning and driving home her message that Mr Obama is a flawed, even unelectable, candidate.

It means continuing to focus over the next month and beyond on the nasty controversies over Mr Obama’s pastor, his comments about “bitter” small-town Americans, his apparent lack of enthusiasm for gun ownership – and just about anything else she can throw at him.

Mrs Clinton may relish her role as an aggressive populist warrior, or “no shrinking violet” as she puts it, but there are also signs that many Democrats are worried at the way this battle is being fought. For all his difficulties over the past fortnight, Mr Obama’s trickle of new super-delegate endorsements has still been bigger than that of Mrs Clinton.

A campaign that once crackled with historic potential has long since ceased to excite or amaze. Mr Obama may have been painted an elitist or even a “pansy” but at great cost to the reputation of Mrs Clinton herself. One poll yesterday suggested that 62 per cent of voters believe she only says “what she wants people to hear”. One Democrat said yesterday: “Fatigue is setting in, a lot of us just don’t want to be in a perpetual war.”

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