Even after all the hype, this Tuesday’s vote in Pennsylvania will be a watershed primary election. This isn’t because it could determine whether Hillary Clinton’s campaign continues on its brutal, nihilistic path towards the destruction of the most promising figure in the Democratic party since Kennedy.

It isn’t because it’s been an age since the last primary vote and every nasty toxin in American culture has been drawn to the surface by the Clinton poultice. It isn’t even because Pennsylvania is an indisputably important and large state that any Democrat needs to win in November.

It is because the Clintons have turned Pennsylvania into a microcosm of what they think the general election will be in November. And the Clintons are running as the Rove Republicans. If they fail to destroy Barack Obama as effectively as Karl Rove – Bush’s master of the dark arts – destroyed Al Gore and John Kerry in 2000 and 2004, with tactics just as brutal but even more personal, then they will have driven American politics to a critical point. They will have shown that the paradigm that has reigned in US politics for at least two decades has been shattered.

That’s what is being tested this coming week. It may be the most important vote in America until the final one in November.

For a month now, Obama has been pummelled by a Democrat in ways I have never witnessed in a primary campaign. Senator Hillary Clinton has directly argued that he is less qualified to be commander-in-chief than the Republican nominee, John McCain. She has said that she doesn’t know for sure that he is not a secret Muslim. She has said his choice of church is unacceptable to her. She has said he deliberately wants many Americans to continue scraping by without health insurance.

Her campaign has insinuated that he was once a drug dealer. Her husband has equated him with the rabble-rousing preacher Jesse Jackson. The Clintons have publicly associated Obama with domestic terrorist William Ayers, with the militant Palestinian group Hamas, and with antisemitic demagogue Louis Farrakhan. And what is remarkable about all this is that most of it was not done by surrogates, but by a former president of the United States against a senator in his own party, and directly by Clinton herself. Every time you think: “Nah, they won’t go there, will they?” – they do.

Right now, in Pennsylvania, Clinton is running only negative advertisements designed to exploit Obama’s gaffe a fortnight ago, when he described some rural Pennsylvanians as bitter, and as “clinging” to some traditional identities because they feel left out of economic and social change. It was a stupid offhand comment, easily misinterpreted, and Obama deserved a hit.

But this is what the Clintons’ actual advertisement says, voiced by several unidentified Pennsylvanians: “I was very insulted by Barack Obama.” “It shows how out of touch Barack Obama is.” “The good people of Pennsylvania deserve a lot better than what Barack Obama said.”

This is a swing state. For the Clintons baldly to coopt exactly the kind of antielitist rhetoric used to marginalise Democrats by Republicans for three decades is to take the campaign warfare to a whole new level of earth-scorching.

For good measure, the ABC News debate last Wednesday night could have been crafted by Rove. For the first three-quarters of an hour, every conceivable personal attack on Obama was aired by the moderators, including former Clinton protégé, George Stephanopoulos.

Obama was asked if his failure to wear an American flag lapel pin at all times was a sign that he didn’t really love America. He was asked if he was an elitist. He was asked if he secretly condoned domestic terrorism, on the grounds that an old 1960s Weather Underground radical had sponsored a fundraiser for him. He was asked whether his former pastor, an ex-marine, was a patriot. And on each occasion, Clinton jumped in to exploit the attacks by the ABC moderators. It was so brutal and unrelenting that you almost looked away.

Obama, moreover, wilted. He didn’t punch back. He seemed completely exhausted, drained, almost detached. I’ve seen him this way before, but never before 10m viewers in prime time. It was his worst performance yet.

In one debate, all the tactics deployed by Republicans since Lee Atwater ran George Bush Sr’s guns-and-flags-and-taxes campaign in 1988 were unloaded on the rookie. Clinton grinned. The next day, her husband said she “did great”. There was almost a liberated sense in the Clinton camp that, finally, they had been able to do to a Democrat what Republicans had done to them for the past two decades: insinuate treason, lack of patriotism, elitist snobbery, countercultural deviance, and every other red-blue hot-button meme that could stroke some electoral erogenous zone somewhere.

Not since the Clintons ran radio ads in 1996, bragging that they had defended American values from homosexuals, had the adoption of pure Republican tactics been so obvious. And this time, it was against a Democrat.

This, the Clintonites tell us, is what the Republicans will do to Obama this autumn. So we’re only showing you! The strategy is to persuade super-delegates that only the Clinton brand can withstand Rove-style attacks, and so foment a revolution before or at the convention to dislodge the candidate with the most pledged delegates and the greatest number of popular votes.

They are, of course, only doing this for the sake of their party, their country and the world. That the tactic also correlates with the Clintons’ recapturing control of a party that was finally moving past them is pure coincidence.

And that’s why Tuesday will be so instructive. Hillary Clinton should win Pennsylvania easily. She had a 20-point lead until relatively recently. And if the Clintons are right about their classic Atwater-Rove tactics, she will win by double-digits after throwing the kitchen sink, the boiler, the couch and the septic tank at her opponent.

However, if Obama keeps her lead to single digits, if he goes on to win in North Carolina and Indiana, if the momentum of the race does not change, something else will be shown.

It will show that the crisis America is in now has made the kind of tactics of the past two decades moot. It will show that the issues of the Iraq occupation, the teetering economy, the unsustainable debt, the collapsing dollar, the constitutional disarray and the moral collapse of the torture programme are now more salient than cultural identity. It will show that the voters actually want to debate something more than lapel pins and who is or is not a secret Muslim or patriot. It will show we are in a new era.

Maybe we’re not. Maybe the old politics and the old patterns have one more turn of the screw to go. Maybe the Clintons are right. And that’s the beauty of democracy. On Tuesday, we will go a long way towards finding out.

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