The United States midterm elections are always important. But the elections on Tuesday matter in ways that few midterm contests can have matched. Yes, it will take more than one election to mend the damaged and angry political mood that, in the last two weeks alone, has seen a fervent Donald Trump supporter send bombs to several Democrats, and a white supremacist commit the most heinous act of antisemitic violence in the country’s history. The man in the White House is not the only thing that must change. But the journey has to start somewhere. You only have to imagine how much more difficult the journey will otherwise be to grasp the exceptional responsibility that rests on the shoulders of US voters on Tuesday.

Donald Trump is not the sole reason why American politics have become so toxic, why Americans’ faith in their institutions has been so shaken, or the influence of the US for good in the world so diminished. In many ways Mr Trump was the product of already existing toxicity, shaken faith and declining prestige. But he has turbo-charged this decline deliberately, as a matter of conscious policy. He seeks consistently to be the president of some of the United States, not of the country as a whole. Against those who do not support or agree with him he deploys only hate and scorn. He lies and provokes as a matter of strategy. This is a president without precedent, and although in the US democracy is strong, it is not indestructible.

Take the issue of voting rights. It is often assumed that the US constitution embodies a federal right to vote. It does not. Voting is administered by the states. Most states are in Republican hands, and the districts that will send members of Congress to Washington this week have frequently been gerrymandered. In many states, including North Carolina and Wisconsin, Republicans have imposed restrictions on early voting, postal voting and voter identification, all of them designed to prevent black Americans from voting. In Georgia, officials tried to close seven out the nine voting places in a predominantly black area on the pretext that disabled access was inadequate.

The US constitution is celebrated for its checks and balances. Yet partisanship is now so entrenched and unbending that institutions themselves are beginning to creak. The White House is in the hands of a lying and rule-breaking racist executive who, apart from all his policy failings, refuses to release his tax returns, blurs the distinction between official and personal interests, meddles in investigations in which he has no business and who deployed thousands of US troops for a purely partisan reason. Meanwhile, since the Brett Kavanaugh confirmation, the supreme court is now more firmly than ever under partisan rightwing control, opening up the near certainty of an attempt to overturn US abortion rights.

So there is a strong constitutional case, as well as a strong political one, for recapturing the legislative branch from its dishonest and sycophantic rightwing Republican leadership. Democratic control of the House of Representatives would constrain Mr Trump by investigating issues that have been shamelessly ignored by the current House leadership. Democratic control of the Senate, a long shot, would clip his wings even more. Democratic failure this week, by contrast, would be – and would be taken to be – an electoral endorsement of Mr Trump.

This is a pivotal election for Americans, for American democracy, and for the rest of the world. Yet it comes at a time of decent US economic growth and high employment, when Republicans are energised, and Democrats are divided about their future course. It is far from guaranteed, in the light of 2016, that Democratic enthusiasm and money will turn into the blue wave that we want. But there is no more important political task anywhere in the world today than to seize this moment.