Donald Trump’s short but indelible political career has been based around the principle of divide-and-fool. His acquittal in the impeachment trial by the US Senate will further fan the flames of the most profound national split since the Vietnam war, perhaps even the civil war.

First, expect Trump to be cocky and take a victory lap, falsely claiming “exoneration” just as he did after special counsel Robert Mueller’s Russia investigation left him bruised but unbowed. A day after Mueller’s leaden testimony to Congress, the president felt able to act with such impunity that he made his bullying phone call to the leader of Ukraine.

Now, we must ask, who will he call a day after being cleared by the Senate?

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Admirers of the US president will take heart that a man many imbue with folk-hero status, who gives them long yearned-for “payback” against a perceived cultural, economic and social elite, has survived yet again. They share his view that he was the victim of “a coup” and embrace his sense of grievance and resentment. Trump supporters interviewed by the Guardian at his recent campaign rally in Des Moines, Iowa, put it down to the latest machinations of corrupt politicians in Washington.

On the other side, feelings run equally high. Democrats and other critics of the president argue, correctly, that in strong-arming Ukraine to investigate a political rival, Trump abused his office for personal political gain and has been allowed to get away with it through a miscarriage of justice. He is, they have said, a president who would be king, undermining rule of law and threatening the very fabric of democracy in America – and around the world.

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Adam Schiff, the House impeachment manager who led the case against Trump in the Senate, spoke last week of a descent into “constitutional madness” as spineless Republican senators huddle behind Richard Nixon’s notorious defence of his conduct in the Watergate scandal: “When the president does it, that means that it is not illegal.”

Schiff told the chamber: “Watergate is now 40, 50 years behind us. Have we learned nothing in the last half-century? Have we learned nothing at all? We are right back to where we were a half-century ago, and we may even be in a worse place because this time that argument may succeed.

“That is the normalisation of lawlessness.”

Comparisons to Watergate are illustrative. The House of Representatives voted 410-to-4 to authorise a formal impeachment inquiry against Richard Nixon in 1974, whereas no House Republicans voted for the Trump impeachment inquiry or for his actual impeachment (Nixon resigned before the sanction could be applied).

John Zogby, a political pollster, noted recently: “The key reason why Nixon resigned was that his polls were at 23%. And there was a famous walk from the Capitol to the White House, where three Republican congressional leaders walked to the White House and said to Mr Nixon, ‘You’re hurting the political party. You’re hurting the Republicans. You have to go.’

“In fact, when Nixon resigned, he said: “It’s clear that I’ve lost my political base. He did not apologise for anything that he was investigated on.”

Trump’s approval rating, by contrast, now stands at 49% in the latest Gallup poll – the highest since he took office and better than Bill Clinton or Barack Obama at this stage in their re-election years. Somehow he is becoming stronger rather than weaker.

A Washington Post column noted five key differences between Nixon and Trump: the current president has a stronger economy, a more loyal Senate, does not face the kind of “smoking gun” tapes that Nixon did, a White House willing to stonewall on his behalf and, perhaps most importantly, Rupert Murdoch’s Fox News is on his side.

Fox’s chief pro-Trump tribune, Sean Hannity, has been savaging the trial nightly. The articles of impeachment “are an affront to our entire constitutional system”, he raged. “No Republican senator – listen, voters out there, you elect these people – should give this one iota of legitimacy.”

Supposedly a 24-hour news network, Fox News has not even shown important chunks of the proceedings: it did not broadcast Schiff’s closing argument. Meanwhile, over on the MSNBC network, there is constant analysis of how a dangerous demagogue poses an existential threat to the constitution.

A report by the Pew Research Center found that among Democrats and those sympathetic to the party, 67% trust CNN, 61% trust NBC and 60% trust ABC, whereas 65% of Republicans and those leaning toward it trust Fox News. “It would be hard to overstate its connection as a trusted go-to source of political news for Republicans,” Pew said.

And it warned: “As the US enters a heated 2020 presidential election year … Republicans and Democrats place their trust in two nearly inverse news media environments.”

Indeed, social media exacerbating the trends, this is going to get worse before it gets better. Polarisation is both symptom and cause, the condition that made Trump’s ascent possible, but which he then exploits and entrenches in an endless and increasingly unbreakable cycle.

Chris Murphy, a Democratic senator from Connecticut, offered a brilliant summary. “This trial in so many ways crystallised the completely diametrically opposed threats that Democrats and Republicans see to the country,” he told the New York Times. “We perceive Donald Trump and his corruption to be an existential threat to the country. They perceive the deep state and the liberal media to be an existential threat to the country.

“That dichotomy, that contrast, has been growing over the last three years, but this trial really crystallised that difference. We were just speaking different languages, fundamentally different languages when it came to what this trial was about. They thought it was about the deep state and the media conspiracy. We thought it was about the president’s crimes.”

Each side therefore regards November’s election as a struggle between right and wrong with the prospect of defeat a cause for panic. Trump’s acquittal will become the latest tool in his kit for polarising the country and firing up his base, joining race, abortion rights, the climate crisis, supreme court justices, dark warnings against “socialism” and a host of other wedge issues. That instinct was on full display in Tuesday’s State of the Union Address, where the emboldened president goaded his opponents with flourishes worthy of reality TV as Republicans chanted “Four more years!”

As the House speaker, Nancy Pelosi, theatrically tore up Trump’s speech, it was a reminder that Democrats will fight fire with fire and have a base of their own to energise. Even supposed moderate candidates such as former vice-president Joe Biden and former South Bend mayor Pete Buttigieg are significantly to the left of Barack Obama on issues such as climate and immigration. These are dark days for technocratic centrism and the “third way”.

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If the presidential election were decided by simple popular vote totals, it would be all over, bar the shouting. Indeed, Trump would have been beaten soundly by Hillary Clinton in 2016. But the quirks of the electoral college mean that many observers have a gut feeling he can pull it off again in key midwestern states.

If he does not, there are some who worry that he will simply refuse to accept the result, returning to old lies he propagated in 2016 about illegal voting. Will Republicans dare to slap down this modern Caesar and remind him of the rule of law?

In this hyperpartisan era, it no longer seems a safe bet. A constitutional crisis over the election result would make even the impeachment trial look like child’s play.

Historian Niall Ferguson wrote the book Colossus: The Rise and Fall of the American Empire in 2004. The American empire, economically and militarily powerful and newly strong rich in oil and gas, is far from fallen. But to future historians, the impeachment “trial” offers a glimpse of what an imploding empire might look like.