Every US election, presidential or midterm, is always hailed as the most important of our lifetimes, at least by the candidates stumping for votes. But this time the hype may actually be justified. The vote that will take place on Tuesday – electing a new House of Representatives, one third of the Senate and a slew of governorships – could not be more significant. To Americans, most obviously, but to a watching world too. It is a referendum on the age of Donald Trump, delivering a verdict on whether the last two years has been a horror show to be repudiated or a model to be advanced and copied.

The stakes are high. If Democrats retake the House, they will be able to slow or stall Trump’s agenda. Even if they stop short of launching formal impeachment proceedings, they will have the power to hold hearings into the myriad corruption scandals that surround the president and his appointees. There will, at last, be a substantial check on his power.

If Democrats wilt, then Trump and Trumpism will be branded with the label that matters to him most: winner

If, on the other hand, Democrats fall short in the House and – as the polls predict – Republicans maintain or even tighten their grip on the Senate, then Trump’s position will be even stronger than when he took office. He will hold the executive and both houses of Congress, alongside what is now, thanks to him and the two appointments he made, a more reliably conservative supreme court.

But the significance of Tuesday goes far beyond the formal allocation of powers. What matters as much, if not more, is whether Trump is boosted or bruised by the results. If Democrats surge, the shock victory he won in November 2016 will come to look more like a freak event, one attributable to an unhappy convergence of a flawed opponent in Hillary Clinton, an eccentric electoral system in which the candidate with the most votes is not necessarily the winner, and a helping hand from Vladimir Putin. If Democrats wilt, then Trump and Trumpism will be branded with the label that matters to him most: winner.

He will claim – and be justified in claiming – that, for all the liberal hand-wringing, he has been vindicated, both in style and in substance. He has campaigned hard, packing in daily rallies across the country, telling the crowds at each stop that a vote for the local Republican candidate is, in fact, “a vote for me”. So if Republicans prevail, Trump can say that Americans saw what the Trump era entailed – and they wanted more of it.

Play Video 2:43 What are the US midterms and why do they matter? – video explainer

The pattern of serial, egregious lying – at last count he had made more than 6,400 false or misleading claims over the 650 days of his presidency, hitting a rate of 30 whoppers a day during the current campaign – would have won public approval. Americans would have accepted the semi-admission of mendacity Trump himself offered this week: “When I can, I tell the truth.” Meaning: when the truth is inconvenient, I lie.

They would also have approved a political approach that has Trump serve not as President of the United States but as president of his base. His electoral gamble is that he does not need to build bridges across the nation’s divides, he does not need to win over the undecided, he simply needs to stir up and mobilise his own supporters. This guiding rule and the habitual lying are connected: he simply says whatever will rile those angry white voters who back him, regardless of truth, regardless of how much damage it will do. A telling moment came recently, when he was asked if he regretted mocking Christine Blasey Ford, the woman who testified that supreme court nominee Brett Kavanaugh had sexually assaulted her. What if Ford was telling the truth? “It doesn’t matter,” Trump said. “We won.”

On Tuesday, US voters will pass judgment on that approach. If Republicans triumph, they will have endorsed it – and proven that, so long as the economy is ticking over nicely, it works. But they will also have validated, even given a mandate to, the main theme on which Trump has chosen to fight this election: immigration or, more starkly, xenophobia and racism.

His closing messages could not have been clearer. In the last few days alone, he has proposed ditching the 19th-century, post-slavery constitutional right of every child born in the US to be a citizen. He has said he will despatch a staggering 15,000 troops to the southern border to beat back the “caravan”, a supposed army of invasion, which in fact consists of a dwindling number of desperate asylum-seekers from ravaged parts of Central America. And he has aired an unabashedly racist and wholly dishonest advertisement that paints migrants as murderers, and falsely claims Democrats want to throw open America’s borders. To say nothing of his reaction to the most lethal antisemitic attack in US history, when 11 Jews at prayer were gunned down in Pittsburgh last Saturday: Trump kept on emitting his dog-whistle attacks on George Soros, a codeword heard and understood by Jew-haters everywhere.

Trump blames possible midterm losses on recent US terror attacks – live Read more

If Democrats do well, the Trump political playbook – which relies on lies and hate – will lose its gloss. Republicans will shift away from it, fearful that they cannot rely on an angry base alone to win elections. But if Democrats falter, the lesson will be clear. Turn your opponents into enemies, dial up the fury and the fear, and power will be yours.

And that lesson will not only be learned in the US. Trump has long been the face of the populist wave that has surged in Europe and which this week gained its most overtly fascistic expression in the victory of Jair Bolsonaro in Brazil. If Republicans defy expectations and hold on, proving that 2016 was no fluke, there will be more wannabe Trumps. Bashing the press, turning on minorities, trashing norms, intimating violence: that will be the formula for success, bottled and ready for export.

But if Trump were to stumble, he would suddenly look less like a model than a warning. Hate-filled populism will be seen to have its limits. The tunnel that we all entered that night two years ago will, at last, give way to a little light.

Americans are lucky: they can choose which outcome they wish to see. The rest of us have no such power. It is our fate to watch and wait – and hold our breath.

• Jonathan Freedland is a Guardian columnist