It's now or never: Clinton needs to win big to keep alive slim hope of nomination

After six weeks without a primary, the Democratic Party candidates battle it out this week with a crucial vote in a swing state that could just break the deadlock. Rupert Cornwell reports

'You got really pummelled." Thus a sympathetic lady at a Barack Obama rally on the morning after the night before – the night before being Wednesday's latest, and certainly most venomous, in the series of 21 candidates' debates that began in early 2007, the mists of prehistory by the standards of this campaign without end for the Democratic presidential nomination.

The lady's comment was no exaggeration. In Philadelphia's National Constitution Center, Obama was subjected to a two-hour battering, first by the questions of the ABC News moderators and then by the barbed answers of his opponent, Hillary Clinton. On Tuesday, however, comes the true gauge of his performance. After six strange weeks without a single primary, Pennsylvania, swing state and unmatched cross-section of the entire country, votes in what has become the most keenly awaited and minutely dissected contest of the season.

Measured by numbers alone, Obama's overall lead has remained pretty stable since the last major votes in early March, when Clinton kept her campaign alive with wins in the Ohio and Texas primaries. In national polls and in terms of delegates, Obama retains a clear advantage. He goes into Pennsylvania with a lead of 1,415 to 1,251 in pledged delegates awarded in the primaries and caucuses so far, and though Clinton has more superdelegates – the unaffiliated party elders who may vote for either candidate – he has a comfortable lead in the overall count, of 1,648 to 1,507.

Given the proportional distribution of delegates, the New York senator has no hope of overhauling her rival in pledged delegates – even if she were to win in all eight remaining contests, ending with Montana and South Dakota on 3 June. By the same token, without superdelegates, Obama has no way of reaching the winning post of 2,025 needed to secure the nomination.

As of Friday evening, 256 superdelegates had publicly lined up behind Clinton, and 233 behind Obama, but that advantage has narrowed considerably. Since "Super Tuesday" on 5 February – the jumbo primary day that was supposed to settle the race but which in effect created the current deadlock – the Obama camp claims to have picked up 80 superdelegates compared with just five for Clinton. Nonetheless, the outcome rests with the 300-odd superdelegates who have not declared for either candidate, despite entreaties from both to do so.

We may not have to wait much longer, however. Howard Dean, the party chairman, is urging the undecided to come off the fence "starting now", to allow the party to turn its sights on John McCain well before the Denver convention at the end of August. In the meantime, the tide of endorsements and grass-roots support continues to run Obama's way. On Friday Robert Reich, Bill Clinton's first labour secretary and a friend of the former president since they were Rhodes scholars at Oxford, became the latest member of the Clinton old guard to come out for Obama. Nationwide, the Illinois senator now has a double-digit lead as preferred candidate of likely Democratic voters.

With its old industrial roots, large blue-collar vote and relatively elderly population, Pennsylvania has long been seen as Hillary Clinton country. In the past month, polls suggest, her lead has shrunk from 15 per cent or more to single digits. For all her efforts, moreover, she seems to have gained little from Obama's now celebrated "gaffe" – when he ventured earlier this month at a ritzy San Francisco fundraiser that working-class Americans "cling" to guns and religion out of "bitterness" at their economic circumstances. In politics, the definition of a gaffe is when someone accidentally blurts out the truth. The electorate seems to have taken the incident in exactly that spirit.

But she is still ahead, and only if Obama scores an upset win on Tuesday will Pennsylvania end the race. Every sign is that Clinton will prevail. The crucial question is by how much. The expectations game is thus all important. The Obama camp will portray a Clinton victory by fewer than five or six points as a moral victory (even though their man outspent her at least two-to-one in the state), and her fundraising may become even more difficult.

A 6 to 10 per cent margin, which most experts expect, would leave matters much where they are now, with Clinton down but not yet out. But a thumping win, by a dozen points or more, would be another matter. Not only would such a result give her campaign a massive intake of oxygen, but it would also confirm what many suspect: that contrary to appearances, the dynamic of the contest has changed.

Above all, a big win would buy her time. Wait, she would be able to argue to wavering superdelegates, consider this new evidence that she, not Obama, would be most likely to defeat John McCain in November. After Ohio (not to mention Florida and Michigan where she won, albeit by default, the now-voided primaries in January), Pennsylvania shows she is better equipped than her rival to win the big swing states.

And she would be making this argument when the bloom is off Obama's candidacy. The San Francisco "gaffe" may be evidence the Illinois senator is out of touch. Or maybe not – if the reaction of an experienced voter in Oil City, in north-western Pennsylvania's remote and Republican Fifth District, is anything to go by. "Even here," she told me, "I wouldn't be surprised if Obama did it; people are just so fed up."

But, as Thursday's debate proved, the days when Obama floated above the fray with his message of hope and unity are gone for good. At last he is being roughed up a bit – the fate of front-runners through the ages. For the first 45 minutes, the questions were entirely about the "gaffe", his failure to wear a flag pin in his lapel, his relations with the fiery pastor Jeremiah Wright and his connections to a former member of the Weather Underground (a radical anti-war group from the 1960s).

An irritated Obama complained, with good reason, at how the moderators put trivial personal questions ahead of policy issues. His supporters were enraged at the apparent bias against their man. But that's politics, and for Clinton, it was an opening: if her opponent didn't like the heat, he should either stay out of kitchen, or prepare for an autumn frying by a Republican attack machine.

With her aggressive and newly negative tactics, Clinton, too, is playing with fire. Her ratings for honesty and trustworthiness have sunk lower than ever before. Her every swipe at Obama merely provides ammunition for McCain. Indeed, the Clinton and McCain critiques of Obama are uncannily similar. He's too inexperienced, they say, weak on national security, and out of touch with ordinary Americans.

No wonder Democratic leaders grow more anxious. Increasingly, they are haunted by the ghosts of failed candidates past. Obama could go the way of Michael Dukakis in 1988, Al Gore in 2000 and John Kerry in 2004. Is the party going to throw away its most winnable election in a generation by again nominating a candidate who may be right on the issues, but is seen by voters as too liberal, too elitist, insufficiently patriotic, and not tough enough to stand up to Republicans who will stop at nothing to hang on to power?

In hypothetical match-ups, McCain is now narrowly ahead of Clinton and has pulled level with Obama. That, of course, is in part because he can play the statesman while his opponents mudwrestle. But it may reflect something far more worrying. Despite a hugely unpopular war and near-certain recession, could this straight-talking war hero be the one Republican who can hang on to the working-class whites who deserted Dukakis, Gore and Kerry?

American presidential campaigns, people say, go on for ever. But with good reason. Obama is still clear favourite for the nomination. But he is still in some respects a little-known figure, who was given a free pass by the media during the early primary season – largely because his national political career has been so short. For the first time this past month, he's been under fire, and Americans are learning how he handles adversity. The answer, in fact, is not badly at all. Yes, he got pummelled. But the politician who's never been pummelled isn't a politician at all.

And so, barring an unlikely Pennsylvania knockout, the battle goes on – to Indiana and North Carolina on 6 May, to Kentucky and Oregon in a fortnight, like Pennsylvania all potential game-changers that may change nothing at all. At least, however, there probably won't be a 22nd debate, much as the Clinton camp would love to have one. Aren't 21 more than enough, the Obama people say. After last week, who can blame them?

For rolling comment on the US election visit: independent.co.uk/campaign08