Donald Trump must be re-elected in 2020. Not because he is the best person to be America’s president – he manifestly is not. Not because his record qualifies him for a second term. The man responsible for the longest government shutdown in history and a reckless and dangerous approach to international affairs has already conclusively proved his unfitness to lead. Trump’s personal conduct shames the country whose people and interests he professes to serve.

All this holds true. Yet it does not alter Trump’s most pressing, selfishly subjective 2020 calculation: that he must win again to be sure of staying out of jail. By convention, sitting presidents are not subject to criminal prosecution, although, legally speaking, it is possible. What is certain, however, is that if Trump loses or does not run next time, he will forfeit all actual or notional immunity from criminal proceedings on 20 January 2021. If and when that point is reached, it will be open season on Trump.

Even by his deplorable standards, the past week has been torrid, but will it prove a watershed moment in shifting Americans’ perceptions of their leader? Despite all the scandals and disgrace of the past two years, despite all the sex, lies and betrayals, despite Trump’s evident contempt for the US constitution and democratic principles, his base support has remained remarkably steady. Roughly two in five voters still back him.

Trump has been dismayingly successful in discrediting much of the criticism that has come his way, by dismissing it as partisan and untruthful. Rather than address a given issue, he typically attacks the motives and integrity of his questioner, thereby offering unwitting insight into his personal psyche. Even the most scrupulous media outlets are accused by him of peddling “fake news”. All this undermines trust and exacerbates divisions, his favoured modus operandi.

Last week’s electrifying congressional testimony by Michael Cohen, who knows Trump better than most, may have finally short-circuited this pernicious cycle of denial. Cohen was Trump’s personal lawyer, enforcer and bag-carrier; the quintessential insider. He knows where the (figurative) bodies are buried. When Cohen, admittedly a convicted perjurer, spoke of Trump’s innate racism, habitual mendacity – and potential criminality – it carried the unmistakable sting of truth.

As a result of Cohen’s testimony, Trump’s opaque business dealings, tax returns, foreign bank loans, property deals, “charitable foundation” and family finances will all now come under closer scrutiny. To take just one example: if Cohen’s claims about “hush money” payments to a woman with whom Trump allegedly had an adulterous liaison are proved, he could face criminal charges of subverting campaign finance laws. Cohen also alleged Trump knowingly engaged in insurance fraud.

It is not merely the newly empowered Democrats who are on Trump’s trail. Any day now, Robert Mueller, the special counsel investigating the Trump campaign’s activities in the 2016 election, will present his findings. Mueller is expected to focus on possible connections to Russian disinformation and electoral subversion operations, the WikiLeaks disclosure of Hillary Clinton’s campaign emails, alleged conflicts of interest and any obstruction of justice arising from Trump’s 2017 sacking of the FBI director, James Comey.

It is a matter of the utmost importance that the Mueller report be published in full; any White House attempt to prevent it must be fiercely resisted. Likewise, voters have a right to know what another, separate federal investigation uncovers. This probe, currently under way in New York, reportedly concerns potentially illegal cash donations to Trump’s campaign by foreign individuals or entities and also to his 2017 inaugural committee, which raised a record $100m.

If Cohen’s damning insider account and the accumulating evidence of multiple instances of past and present wrongdoing are not yet sufficient to sow terminal doubts among Trump’s voter base, then perhaps its fracturing will be hastened by last week’s other presidential meltdown – Trump’s disastrous mishandling of his nuclear summit in Hanoi. Trump had a unique opportunity to cut a deal with Kim Jong-un, North Korea’s dictator, that, unusually, would be backed by China, Russia and the west.

Just about everybody would like to see curbs on Pyongyang’s nuclear arsenal. Trump’s strongest card was the recognition such a summit affords Kim. Yet, for a second time, he gave him that prestigious boost for free, hailing him as a friend and “great leader”. Then Trump fumbled the talks and flounced off petulantly in trademark puerile style. Adding insult to injury, he absolved Kim of personal responsibility for the death of an American student, Otto Warmbier, an unjustifiable sop for which he has since been rightly condemned.

Elizabeth Warren, a Democratic presidential hopeful, warned in Iowa last month that by the time the 2020 election arrives, Trump “may not even be president... in fact, he may not even be a free person”. Her remark pointed to the possibility of impeachment, while Trump remains in office, and future criminal prosecution. Some said Warren was joking. She probably wasn’t. Trump’s parody of a presidency is no joke – and on the evidence of recent days, the worst is yet to come.