At the Shriner Temple in Indianapolis, the giant helium balloons spelled out HILLARY as Tom Petty belted out: "I won't back down" on the sound system. "Tonight we came from behind. We have broken the tie and thanks to you, it's full speed on to the White House," the woman aspiring to be the first female US president told her cheering supporters.

Two months ago in these columns, I sang the praises of this comeback queen.

Whatever you think about her personally, it's hard not to admire her grit.

The abortive attempt to reform healthcare, the Arkansas investigations, Bill's affairs and attempted impeachment: this woman had bounced back more times than a champion trampolinist. Surely this is the sort of staying power the Democrats need to beat John McCain to the Oval Office? This week's was just the latest do-or-die contest. She has won against the odds in New Hampshire, Texas, Ohio and Pennsylvania, and yesterday by a narrow squeak she took Indiana.

I've always found Hillary Clinton more admirable than likeable. As a woman of her generation, I find it hard to resist the notion of a female president and muse on how Bill might fit into the arrangement. Would he be called "the First Gentleman"? Seems unlikely. Would he be content to play a Denis Thatcher sort of role, smiling politely in the background before sneaking off to play golf with his chums? There's no doubt that she would have an even higher mountain to climb than British female aspiring politicos, such is the machismo swirling around American politics. (For all her current woes, Wendy Alexander has never had to cope with sexist hecklers shouting "Iron my shirt!") However, as I wrote on March 6: "If Hillary ends up being beaten by a campaign of sheer genius from a man who seems born to play to the gallery, then fair enough." The narrowness of her win in Indiana and Barack Obama's simultaneous thumping victory in North Carolina combine to rob her of what she predicted would prove a "game-changing moment". She was careful not to repeat the phrase yesterday (although he did), because it was indeed a turning point, but not in the way she had hoped.

In recent weeks, the Democratic campaign has taken a nasty turn and it is partly her fault. To differentiate herself from her opponent, she has taken a weapon in each hand: race and class. In fairness, Barack Obama does have questions to answer on the Reverend Jeremiah Wright affair. It is not possible to believe that Obama had not heard the Chicago pastor expressing narrow incendiary views, offensive to white Americans, unless he has been exaggerating his church attendance record. He loses either way.

Oprah Winfrey stoked the controversy again on Monday when it emerged that she had sampled the rhetorical wares of the Rev Wright many years ago and decided not to buy. Mrs Clinton's campaign team may not have been responsible for placing cleverly-edited soundbites from his rabble-rousing sermons on YouTube, but they made a number of heavy-handed attempts to capitalise on them, giving the campaign a racial edge for the first time.

She must have known the impact this would have, especially in Indiana, northern stronghold of the Ku Klux Klan.

On March 18, Obama struck back with perhaps the finest speech of his life.

Without denying his personal links with Wright, he firmly distanced himself from the pastor's inflammatory opinions. In words that would not have been out of place coming from John F Kennedy in their power and subtlety, he spoke of "the brutal legacy of slavery and Jim Crow" and "the complexities of race that we have not yet worked through". The subliminal message was that this mixed race candidate was the one best placed to heal past divisions and confront the big challenges of the fragile global economy and climate change.

The problem for Obama is that once he was labelled as "the black candidate", race came to frame the argument. It boxed him in. Even his rather clumsy references to his white mother and grandfather (a Second World War veteran, buried with the stars and stripes draped over his coffin), could only mitigate the damage. If, as seems increasingly likely, he lands the nomination, Clinton could be blamed for racialising the campaign in a way that weakens the Democratic cause.

The same applies to class. In her pursuit of the white working-class vote, it has suited the Hillary camp to portray Obama as an Ivy League elitist.

It is with this group that Obama is still struggling. Snobby-sounding remarks about small-town narrow-minded bitterness may be true but didn't help him. And his attempts to empathise with blue-collar workers as the main victims of the manufacturing turndown, outsourcing and the credit crunch have had limited effect, despite the close result in Indiana. Again, they are going to be important, especially in the key swing states such as Ohio, Michigan and Pennsylvania, if the Democrats are going to win the presidency. The danger in the current situation is that Clinton has been so effective at holding this group that if Obama takes the nomination, McCain will benefit.

The biggest significance of this week's results is that they demonstrate that the Illinois senator is capable of surviving what one political journalist described as "the s**t storm". After Pennsylvania, the biggest lingering doubt about Obama was how well he would stand up to close scrutiny and downright hostility. Now we know that when the going gets tough, this man gets going. He has huge support from young black Americans and the liberal intelligensia. He is also far better placed to win over independents - disenchanted Republicans would rather eat their young than vote for a Clinton.

The Democrats deserve to win in November. The Republicans have dragged the world into a disastrous war and left the American economy in tatters. The maths are now stacked against Hillary Clinton, virtually irreversibly, unless she runs off to the courts in a desperate attempt to retrieve lost delegate votes from Florida and Michigan (where Obama wasn't even on the ballot). Postponing the decision until the Democratic Convention in August would leave the chosen candidate barely two months to mount a general election campaign against a Republican who sewed up his nomination in March.

Obama's North Carolina victory speech sounded like a nomination acceptance speech with its talk of how "the party of Roosevelt and Kennedy" would need to take on the Republicans. He has shown he can win a big swing state and he has the money, compared with Clinton who had to donate $6.4m to herself.

Democratic donors would surely prefer to give their dollars to the White House race, not this one.

The comeback queen needs to realise that she can still run but only into the ground. Her admirable sticking power now looks like a stubborn refusal to give up, even when the writing is on the wall in ten-foot-high capital letters. It reads: THE END.

anne.johnstone@theherald.co.uk