As Obamamania sweeps the land, the end may be nigh for Hillary Clinton. Her husband has effectively declared that she must win Tuesday's votes in Texas and Ohio to survive. Barack Obama has turned her once-giant leads in those states into dead heats. Even if she manages to win both contests, only implausibly large margins of victory will narrow Obama's current, nearly insurmountable delegate lead.

Yes, Hillary Clinton is a survivor and she may again stun the world just as she did in New Hampshire. But her prospects are grim. Thus it's impossible not to begin imagining what is likely to come next: a general election campaign between Barack Obama and the Republican nominee apparent, John McCain.

Giddy over their man's incandescence - 'He can cure cancer with a simple touch; summon rain in the desert; make peace between black and white, Arab and Jew, Jagger and Richards' - some Obama followers are anticipating a rout. They see their man as a young tribune of hope and prescient Iraq war opponent facing down a cranky old Washington fixture who supported both the war and the failed presidency of George W Bush. Could this be, to borrow a phrase from the Bushies, a slam dunk?

Not so fast. A new opponent and a new brand of voters will both put Obama to his greatest test yet.

Although Hillary Clinton once appeared unstoppable, in hindsight it's clear that she provided Barack Obama with a near-perfect foil. The people who vote in Democratic primaries are a fundamentally idealistic lot. They romanticise politics and want to believe that great dreams can be achieved. In recent years, they have simmered with frustration that their party's leaders have become hyper-calculated sellouts who answer to corporate interests and cow before the Republican attack machine.

With her extreme image-control, her history with shady campaign donors and, above all, her 2002 vote to authorise the Iraq war, she embodied much of what Democratic primary voters hate. She has compounded these problems by dismissing Obama's thrilling vision. While he has promised to transform Washington, Clinton has argued that it takes an insider to operate its vulgar machinery. She has seemed a champion of compromise and Obama the personification of idealism.

Critically, this contrast between Obama and Clinton has largely been one of style and not substance. With few policy differences to debate, their contest has focused on image. For Hillary - older, less charismatic, tainted by past scandals - that dynamic has been crippling.

An Obama-McCain match would be a very different story. The men have fundamental differences on everything from national security to abortion rights, from gun control to health care and taxes. 'It is only natural that when policy differences are smaller in the primary, you focus on the people,' says Dan Schnur, a former top aide to McCain. 'The general election will be about issues.'

Obama is not the policy lightweight that some critics imagine. But as he has demonstrated in countless debates, he is not at his best when discussing issue specifics. He needs grand themes to make himself grand. No issue will test him like McCain's favourite subject: national security.

Clinton has tried, and largely failed, to raise doubts about Obama's ability to keep America safe. But she was severely limited on security issues by the Democratic primary dynamic. She knew liberal voters wouldn't stand for Karl Rove-like attacks on Obama as 'soft' on security. She repeatedly claimed to be the most qualified candidate to lead the country, but always stopped short of flatly declaring Obama dangerously unqualified.

McCain will have no such reservations. He will be free to attack Obama as unprepared for the job of commander in chief. And he may succeed where Clinton has failed. A late-February Los Angeles Times/Bloomberg poll found that voters believe McCain has 'the right experience' over Obama by a 53-22 margin. This is not the portrait of an electorate ready to be swept away by memories of Robert F Kennedy. In the face of doubts about his core qualifications, Obama can't fall back on soaring oratory. He'll be forced to spend more time proving his serious bona fides in ways more likely to elicit yawns than the fainting spells of his recent rallies.

Take Iraq as an example. McCain's strong original support for the war is a political handicap. But public opinion remains divided about the next step in that country and McCain has benefited from perception that the troop 'surge' he backed has succeeded. It is not at all clear that Obama's call for a near-total troop withdrawal will trump McCain's arguments for avoiding 'defeat' in Iraq.

That same poll showed voters trusting McCain to handle Iraq over Obama by a 47-34 margin. The debate will be sombre and perhaps bitter. And it will demand complex strategic reasoning, not inspirational slogans such as: 'Yes, we can.'

There's also the risk of hope fatigue. American culture is absurdly fickle and the media love to defile their own holy icons. And euphoric puppy love must yield to mundane reality: 'People can't be awed by every speech for the next eight months,' says one Democratic Obama admirer in Washington. 'Everyday tedium might do in "hope" more than the Republicans will.'

Finally, McCain may challenge Obama with his own version of hope. Clinton erred badly by treating Obama's vision for political reform as naive. But rather than defend the status quo, McCain will counter with change credentials that cannot be dismissed. In 2002, he passed the most important reform to the federal campaign finance system in 30 years and has engendered bitterness among his Washington colleagues for exposing wasteful spending projects. McCain boosters are already highlighting this. 'One candidate has fought for change his entire life while the other has talked about it,' says one longtime McCain adviser.

By no means will this election be easy for McCain. Iraq could plunge back into chaos and the US economy could spiral further down, two scenarios that would badly stigmatise his party and, quite likely, his candidacy. Obama has also been underestimated before. Few people imagined he could defeat Clinton in 10 straight primary contests as he has this winter or raise up to $2m per day.

For the moment, Obama remains the most inspirational political figure in a generation, but his admirers must prepare themselves for a reality check. Winning the Democratic nomination, if he can pull it off, will have been the easy part.