She was the prohibitive front-runner, the over-confident front-runner, then suddenly the underdog. She was the Comeback Kid, the underdog again, and, for the past two months, the Senator reinvented as street fighter. Until Tuesday Hillary Clinton's latest political persona had served her well. As the candidate who could not quit, she won primaries in Texas, Ohio and Pennsylvania, and credence for her claim to be a serious contender for the Democratic presidential nomination even though the numbers were already stacked against her. By last week she was earning plaudits from across the political spectrum for sheer stickability - or, as one union leader put it, for “testicular fortitude”. So it will be hard for Mrs Clinton to do what she promised not to do, and quit. But quit she must. After losing heavily to Barack Obama in the North Carolina primary, and beating him only narrowly in Indiana, she needs an unforeseen disaster to strike the Obama campaign to stay in contention. Otherwise she is just spending money (most of it now her own) on damaging her fellow Democrat.

Mr Obama seized the lead in this historic race in February and has shown his own brand of fortitude to keep it. He deserves the chance to mount a serious White House bid, and after eight tumultuous years will find himself the conduit for the yearnings of a huge constituency ranging from left-wing anti-war activists to disaffected Republicans. After a more traditional nomination process Mr Obama might already be scenting ultimate victory. Yet his campaign against the formidable Senator McCain has barely begun, and is already looking vulnerable.

None of Mrs Clinton's reasons for fighting on stands up to scrutiny. Until Tuesday she enjoyed a narrow lead in the total popular vote. No longer. She claims she might regain it if the Michigan and Florida primaries were included in the count, but Mr Obama did not campaign in either state since both had been excluded for breaking party rules. She argues, finally, that party “super-delegates” should back her as more electable than Mr Obama against Mr McCain.

This misses two vital points. Many Democrats who told pollsters on Tuesday that they would defect to Mr McCain if their candidate was not nominated will ultimately put party before personality and vote Democrat in November. Secondly, Mr Obama's strong showings in North Carolina and Indiana followed the best two months of Mrs Clinton's campaign, and the worst of his own. He has been battered but not beaten by his ties to the incendiary Rev Jeremiah Wright and by his own glib criticism of “bitter” Midwesterners. Mrs Clinton, meanwhile, has proved herself a fighter but not a winner. She needed the last two big primaries to embolden remaining undeclared super-delegates to back her overwhelmingly. They show no sign of doing so.

Last year the nomination was Mrs Clinton's to lose, and the US began to contemplate the prospect of its first woman president. She could still be that president, but only if she withdraws now to limit the damage she is doing to her party and the Clinton brand, and runs again in 2012, when she will still be only a sprightly 64 years old.

This year, barring a sudden implosion of the Obama campaign, the US and the world will witness instead the duel between a Vietnam War hero and an African-American young enough to be his son. Mr Obama is not the first candidate to promise change, but he is the first to embody it so vividly. His real campaign has begun.