As in all the best horror movies, at first glance everything looks normal. It’s a classic scene of the American autumn: campaign rallies outside community centres, battle buses emblazoned with candidates’ smiling faces, kids wearing badges and holding up signs, while TV screens fill with debates, punditry and an endless loop of focus-grouped ads. Even the predicted outcome of Tuesday’s US midterm elections fits a template as familiar as falling leaves. Most experts agree that the Democrats will take a hit, losing control of at least one or perhaps both chambers of Congress, because they are the incumbent party – and incumbent parties almost always suffer in midterm – and because times are unusually tough. Inflation, interest rates, petrol prices, fear of crime: they’re all up. Couple that with a president set to turn 80 this month whose approval ratings have often plumbed the depths, and all the elements are in place for the Democrats to take a midterm beating, losing ground even in states they once counted as solidly their own.

But look closer and you see something else. Because next week’s results will decide more than just whether the red team or the blue team takes control of the House of Representatives and the Senate, on which hangs Joe Biden’s ability to get things done. Next week’s elections will also help determine whether, and for how much longer, the US will remain a genuine democracy.

It sounds hyperbolic and that, too, is an American tradition. Candidates always tell the crowds, “This is the most important election of my lifetime” and plenty will have heard Biden’s warning, delivered on Wednesday, that democracy itself is on the ballot in that same spirit. They will have assumed that when the president said, “In our bones, we know democracy is at risk” it was so much campaign talk. But Biden was scarcely exaggerating.

More than 370 Republican candidates for some of America’s highest offices have joined Donald Trump in his big lie of election denial, either casting doubt on or wholly rejecting the legitimacy of the 2020 presidential result. That means a majority of Republicans running for those key positions refuse democracy’s most basic act: accepting the verdict of the voters.

It’s comforting to pretend they’re doing it solely to soothe Trump’s ego, to avoid angering him by conceding that the ex-president lost to Biden fair and square. Keeping Trump sweet is a necessary tactic in a Republican party where he remains the dominant figure, reportedly set to launch another presidential bid later this month, whose endorsement or disapproval is enough to make or break a career. But Republicans’ election denialism is not confined to the past; it applies to the future, too. Several of the party’s candidates have refused to say that they will accept the outcome of Tuesday’s vote should they lose. “I’m going to win the election, and I will accept that result,” is how Kari Lake, would-be governor of Arizona, puts it. Some might spin that as mere election eve bullishness, but without losers’ consent democracy cannot function.

More sinister still, several of these democracy deniers are running for the very state-level posts that will oversee and certify future elections, including the presidential contest of 2024. And they are brazen in their admission that they will abuse the powers of those offices to boost their side and shut out their opponents. “Republicans will never lose another election in Wisconsin after I’m elected governor,” is the promise of one Tim Michels, who seeks to lead that state – and it was not a promise that he would be popular. It’s worth recalling that it was in Wisconsin two years ago that a group of Republican office holders moved to ignore the democratic choice of that state’s voters, who had backed Biden, and instead declare Wisconsin for Trump in the electoral college. If they were to try that trick again in 2024, they might have an ally in the governor’s mansion.

If all of this seems too abstract, consider the Republicans’ new attitude to political violence. Once it would have been a matter of bland consensus that no political objective should ever be secured by brute force. But only a handful of Republicans could bring themselves to hold even that fundamental position following the storming of Capitol Hill and the attempted insurrection of 6 January 2021. The rest refused to vote for the impeachment of Trump for his role in fomenting that violence and, if they condemned the rioters themselves, it was usually in terms qualified and mealy-mouthed.

We’ve seen it again in the last week, after the vicious assault in his home of the 82-year-old husband of House speaker Nancy Pelosi. Paul Pelosi had his skull fractured with a hammer, the alleged assailant a man whose head had been filled with far-right shibboleths including the supposedly stolen election of 2020. And yet the likes of Lake saw the attack as a laughing matter, while other Republicans (and their ally Elon Musk) concocted or spread conspiracy theories that cast doubt on the attack. This in an era when recorded threats against members of Congress rose tenfold in the five years after Trump was elected in 2016.

The Republican party’s shift away from democratic norms is no longer confined to one man, even if he embodies it and accelerates it. It is embedded in the ethos of the party now. Reversing that trend is a daunting prospect because of another shift, one that has been apparent for a while but which is taking especially vivid form in these midterm elections. It is the polarisation of information, so that Americans now exist in two distinct spheres of knowledge, each one barely touching the other.

I witnessed it for myself this week, as I covered an especially intense senate race in Georgia. News came that, in a previous age, would have been devastating for a candidate. A second woman stepped forward to say, on camera, that the staunchly anti-abortion Republican Herschel Walker had pressured her to have an abortion and had paid for it. Yet when I put that news to Republicans gathering at a Walker rally in Madison, not one of them was fazed by it. They just assumed it was the false concoction of the “mainstream media”.

This poses its own danger for democracy. Because there can be no collective decision-making – which is what democracy amounts to – without a collective, agreed-upon basis of facts. If we can’t first agree that the house is on fire, we can’t begin to talk about putting out the flames.

Whatever the outcome on Tuesday and in the long days of counting that may follow, this is a moment of peril for the United States. The world’s most powerful democracy is losing the reflexes and habits that make democracy possible. And, as in all the most terrifying horror movies, the threat is coming from inside the house.

• Jonathan Freedland is a Guardian columnist. Listen to his Politics Weekly America podcast here