America’s midterm elections are principally for the House of Representatives and the Senate, but they are usually regarded as a national referendum on the performance of the sitting president – and an important indicator of how the next presidential election may turn out. That’s especially true of polls held, as with Donald Trump on Tuesday, held during a president’s first term.

Historically speaking, the party of a first-term president has lost seats on all but two occasions since 1789 – sometimes disastrously, as happened to the Democrats in 1994 during Bill Clinton’s first term. In 1858, President James Buchanan’s Democrats were crushed by Abraham Lincoln’s newly formed Republicans, a fracture that opened the way for the American civil war.

While opinions differ over the prospect of a civil war#2 under Trump, there are other, objective reasons for believing he and the Republicans are heading for a drubbing. Trump’s personal approval rating stands at around 41%, according to the latest Gallup survey, well below the average, for this stage in a presidency, of 52%. Only one in three voters feel the country is heading in the right direction.

In findings mirrored by other polls, the performance of the Republican-controlled Congress also gets a resounding thumbs-down, with only 21% expressing satisfaction. Most tellingly, high public confidence in the expanding US economy – normally the top issue for voters – has not translated into greater approval for Trump.

Most estimates suggest the Democrats are on course to gain the 23 House seats they need for overall control. Of 33 seats judged to be “toss-ups”, 29 are Republican-held. The Senate, where the Democrats require a net gain of two seats to win control, is more finely balanced.

But if the political world has learned anything since Trump’s shock victory in 2016, it is that traditional indicators must be taken with a barrow-load of salt. Trump does not behave like a conventional politician, for the simple reason that he is not one. His midterm campaign tactics have surpassed, in crudeness, the worst excesses of his most cynical White House predecessors. He has appealed unashamedly to voters’ most primitive instincts, primarily fear – and nobody yet knows how those voters will respond.

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

Tuesday’s elections have thus come to represent a seminal moment in the life of modern America. The results will give a broad insight into whether Trump’s narrow 2016 triumph was an aberration, a blip or an unintended electoral accident. Or, alternatively, whether decisively large numbers of Americans really meant it when they backed Trump then, and want more of the same now.

The polls will afford a glimpse of the sort of country the US has become. Set against the overwhelming weight of historical precedent, political experience and poll predictions, logic suggests Trump and the Republicans should lose on Tuesday, possibly big time. But will they?

The choice appears stark. On the one hand, there is Trump’s not- so-subliminal racist, white ethno-nationalism, his genius for division and distrust, and his simplistic us and them narratives. The worst of Trumpism was on show after the Charlottesville white supremacist riot last year. It resurfaced after last month’s pipe bombs and Pittsburgh synagogue murders, in his attacks on the media, and in his scurrilous scaremongering over immigration, symbolised by the US-bound migrant caravan.

Columnist Paul Krugman said Trump’s gross behaviour accurately represented an “increasingly … far-right [Republican] party” that avoided discussing actual policy. “Trump’s frantic attempt to make [the election] about scary brown people rather than healthcare or tax cuts is cruder than anything we’ve seen for a long time, but it’s not fundamentally out of character.” Trump’s whole strategy was one of “maximum ugliness”.

On the other hand, the problem for Trump’s critics and the many voters who, polls say, are tempted to switch sides – suburban voters, women, young voters, college-educated voters, and Latino voters repelled by the president’s divisive antics – is that the Democratic alternative is less than wholly persuasive. The party, after Barack Obama, lacks a leader and is running against the backdrop of a booming economy.

Democrats have also struggled to identify domestic issues with universal relevance that are not to do with flag-waving nationalism, race, ethnicity and faith – the Trump agenda. They may belatedly have done so with healthcare, specifically their defence of Obama-era insurance protections for people with pre-existing conditions. Such protections would almost certainly disappear if the next Congress is Republican.

'Don't boo. Vote': parties draw biggest names to boost Wisconsin turnout Read more

Author EJ Dionne detects a yearning among voters for an end to the vicious polarisation encouraged by Trump. “On the ground, Democratic candidates … are insisting that the country is exhausted by acrimony … and by the evasion of the day-to-day issues – healthcare, education, job training – that they believe most Americans want their politicians to grapple with,” Dionne wrote.

Nationwide, that’s an untested proposition. And the results of the midterms could widen America’s divisions. If the Republicans win, the question will be: how on earth can the Democrats prevent a second Trump term? If the Democrats win, take over Congress, and Robert Mueller’s expanding FBI investigation proceeds unhindered, the question will be: what might a desperate, sinking Trump do to save himself? The mind boggles.