FOR THE past seven weeks, a real-life trailer for November's general election has been showing in Pennsylvania. Barack Obama, the young male lead, once an inspirational blank slate, is now a flawed, morally complex character. His nemesis is tougher, better prepared and more American than he is. The imaginary voiceover might ask: "who can you trust?"

The Democratic primary in Pennsylvania on Tuesday is unlikely to settle anything, but its long, overheated build-up has been a sneak preview of the feature presentation to come.

There is only one way the primary can be decisive. An Obama victory would prove he can succeed in unfavourable territory, demolishing Clinton's claim to superior electability in swing states. She would soldier on, no doubt, but it would be a hollow gesture of defiance. If Obama takes Pennsylvania, he will surely take North Carolina and Indiana too.

On the other hand, if Clinton wins by a landslide, the fight will spill on to the floor of the Democratic convention in August. A margin in double figures, as resounding as her triumph in Ohio, would raise serious, legitimate doubts about the viability of Obama's candidacy.

Unless recent polls, which variously assess Clinton's lead at anywhere between 14% and 3%, are even further off the mark than the telephone surveys that inaccurately predicted Obama would carry New Hampshire, neither of these scenarios will play out. The most likely result is that both candidates will pose as winners, while the Republicans gloat about the glaring weaknesses that have been exposed.

The game in Pennsylvania is expectation management. In press releases and conference calls, the campaigns have defined what constitutes a win on their own terms, to increase their chances of declaring victory, whatever the actual delegate count or popular vote. Privately, Obama has been telling donors that defeat by anything less then 10% would be a successful return.

Pennsylvania should be Clinton country. With the exception of Wisconsin, the broad demographic coalitions the candidates have assembled have held steady throughout the primary season. African-Americans, students, first-time voters and college-educated whites favour Obama, while pensioners, Hispanics, middle-aged white women and blue-collar workers are more likely to vote for Clinton. Pennsylvania's population is older, whiter, poorer and less educated than the national average, much like Ohio, which borders it to the west, where Clinton beat Obama 54% to 44%.

Clinton's father, Hugh Rodham, was born in Scranton, a mining town in northeast Pennsylvania. She has the support of the state's most influential politician, governor Ed Rendell, who has put his formidable patronage network at her disposal. In January, she had a 20-point lead in the polls. Obama's communications director Robert Gibbs told MSNBC's breakfast news programme "we are the underdog in Pennsylvania".

On the same show, Clinton's chief strategist, Howard Wolfson, observed that Obama is "spending more money on TV this week than anyone in the history of Pennsylvania politics ... doing everything he can to win." Viewed in this light, squeaking home by a few thousand votes would justify Clinton fighting on to North Carolina, Indiana and beyond in the hope of fresh revelations damaging Obama.

The contest has long ceased to be about pledged delegates. Congressional Quarterly published a detailed prediction for Pennsylvania, based on existing demographic voting patterns. It concluded that Clinton's net gain in the districts would be three delegates. Winning the popular vote by 10% could be worth as many as a dozen more, distributed state-wide.

Obama's lead is around 160 pledged delegates. Even if Clinton wins every single one of the remaining primaries with an unthinkable 60% of the vote, she would still trail by 50 going into the convention. Her only hope is to persuade super-delegates that Obama is a weak presidential candidate who would be crushed by McCain.

Pennsylvania's four million registered Democrats, in particular the kind of working class white voters Obama has struggled to attract, have become a test case for his candidacy. Every vote counts, not because of its impact on a delegate battle that Clinton has already lost, but because of what it says about the limits of Obama's appeal. A failure to make inroads, despite spending a fortune on advertising and campaigning hard, in person, for more than a month, would suggest the existence of a fundamental barrier that he might find difficult to overcome in November.

It would not be for want of trying. Obama toured the state by bus, mainly stopping in small towns like Wilkes-Barre, Lancaster, Allentown and Burnham. At gatherings in schools, bars, hot-dog restaurants and petrol stations, carefully staged to create the illusion of spontaneity, he laid out a manifesto tuned to the needs of mining and manufacturing communities in decline, explicitly linking foreign policy with cuts in social services: "We spend $18 billion a month in Iraq. What else can we spend it on?"

In Altoona, Obama went bowling, throwing his first two efforts into the gutter, but with Pennsylvania senator Bob Casey at his side, just acting like a regular guy rather than an aloof, Harvard-educated lawyer was enough.

Casey is a socially conservative, economically populist Irish Catholic from Scranton whose father was governor of the state. Of all the endorsements that have come Obama's way in Pennsylvania, his is probably the most important, although it can't hurt to have the owner of the Pittsburgh Steelers American football team on board, or New Jersey native Bruce Springsteen, who posted on his website that Obama "speaks to the America I've envisioned in my music for the past 35 years".

When Obama drank a bottle of Yuengling, brewed in Pottsville, PA, he checked with a fellow drinker that it wasn't "some designer beer or something". Like so many of his attempts to curry favour, it was utterly transparent, but the signs are that it worked. Clinton's formidable lead in the state dropped into single digits.

Then came Bittergate. At a fundraiser in California, Obama told an invited audience (and unwittingly a "citizen journalist") that many Pennsylvanians are "bitter" and "cling to guns or religion or antipathy to people who aren't like them". The comments were picked up on by the mainstream media and Clinton herself. "I was taken aback by the demeaning remarks senator Obama made about people in small-town America," she said. "His remarks are elitist and out of touch."

Elitist is one of the dirtiest words in American politics. Republicans have successfully applied it to a parade of cerebral Democratic losers that begins with "egghead" Adlai Stevenson and ends with John Kerry windsurfing and John Edwards paying $400 for a haircut. But will it stick to Obama? Neo-conservative ideologue and New York Times columnist Bill Kristol immediately used it to compare him to Karl Marx.

On the campaign trail in Indiana, Clinton followed her "out of touch" criticism by reminiscing about the day her father taught her how to shoot. Later, she played up her blue collar credentials at Bronko's bar in Crown Point, chasing a shot of Crown Royal whisky with a mug of Heileman's Old Style lager.

The irony of a woman who has just declared tax returns of $109 million calling her upper-middle-class opponent elitist is not lost on struggling Pennsylvanians. There has been no significant shift in the polls. On left-leaning blogs, for every post calling Obama arrogant or tactless there is a reply from small-town America, saying that despite the clumsy phrasing, he was telling the truth.

Thisletter to Salon.com from a navy veteran calling himself Mjwycha is a typical example: "Manufacturing jobs are gone. Small towns like mine (Mechanicsburg) are rotting from the inside out ... The squeeze is tangible. We ARE pissed off, we are economically insecure, and have been for years. Of course my neighbours have turned inward."

Wednesday night's debate on ABC showcased lines of attack that will be used against Obama if he wins the nomination. Barring one question about Clinton's Bosnia fabrications, he spent the first hour on the defensive. He was questioned about his failure to wear a flag pin, his pastor's anti-American sermons and his connection to former Weather Underground terrorist Bill Ayers.

His replies blamed the political system itself. "The problem that we have in our politics," he said, "is that you take one person's statement, if it's not properly phrased, and you just beat it to death."

Obama's claim to transcend the muck is a key aspect of his campaign, but he risks being labelled a hypocrite. He has been quoting McCain's "100 years" comment about the occupation of Iraq, completely out of context, at every opportunity.

Clinton, for her part, is wearing out the excuse that her attacks are a battle-hardening exercise. "I know senator Obama's a good man, and I respect him greatly," she said, "but I think that this is an issue that certainly the Republicans will be raising." Obama countered: "I can take a punch."

In the last two months, the number of Democrats who see the race as "mostly negative" has increased from 27% to 41%. Three times as many blame Clinton as Obama. In the same Washington Post/ABC poll, Clinton's reputation for telling the truth dropped to a new low - only 39% of the people polled considered her "honest and trustworthy". Favourability ratings have slipped for both candidates but, again, Clinton has suffered most.

The accepted wisdom is that Clinton and Obama's undignified brawl has drastically undermined their party's chances. Conservative commentators have been quick to pour salt into the self-inflicted wounds. Michael Goodwin of the New York Daily News expressed "a growing belief that neither can win the general election in November. It's a problem Clinton has had all along, and Obama, despite being the front-runner, is now proving he belongs in the same soup."

McCain has certainly benefited from a lack of scrutiny while the media focus has been on the Democrats. He got away with confusing Shi'ite with Sunni Muslims three times, repeatedly saying that Iran was supporting al-Qaeda in Iraq. The tax returns which conveniently ignored his wife's vast fortune were barely reported on. But the flipside is that events to define his candidacy, such as his biography tour and trip to Europe and the Middle East, also received scant attention.

Turnout in Pennsylvania's primary will be historically high, as it has been in every Democratic contest so far. 161,000 new Democrats have signed up in the state since November, while Republican registration has declined. On Thursday night, Obama drew his biggest crowd yet - 35,000 people in Philadelphia's Independence Park. So while the memorably nasty exchanges of the past few weeks have undeniably damaged both Democrats, it is too soon for the customary pre-election panic to begin.

Clinton and Obama have bruised each other badly, but they have also relentlessly hammered the message that McCain means more of the same. In Pennsylvania, a failing economy and a deeply unpopular, hugely expensive war must give whoever is nominated more than a fighting chance.