Donald Trump has spat out so many insults and broken so many taboos that it’s hard for any single remark to linger long in the memory. Nevertheless one line from his 2016 election campaign has endured, partly because it was a jaw-dropper and partly because it offered an early glimpse of what would later be revealed as a deep truth about both his candidacy and his presidency – and even our current world.

It came when Trump was praising his early supporters as “so smart” and so loyal that “I could stand in the middle of Fifth Avenue and shoot somebody and I wouldn’t lose any voters”. The imagery was gross, the vanity egregious but, as so often with Trump, his reptilian intelligence had landed on something that was, though ugly, true: namely, that he enjoys a rare form of impunity.

The first proof was victory itself, won despite scandals and revelations that would have felled more mortal candidates. There was his voice on tape, clear and unmistakable, bragging about grabbing women “by the pussy” – and he won anyway. Now, three years on, Trump’s impunity is being tested again, with the launch this week of the public phase of the House intelligence committee’s impeachment inquiry.

Even before the first witness had opened his mouth, Democrats had laid out what should be a slam-dunk case for the removal of a president. Americans have known since September that Trump withheld foreign aid from Ukraine as he pressured its president to reveal, or make up, dirt on Trump’s would-be 2020 opponent, Joe Biden.

You can call that soliciting the interference of a foreign power in a US election. You can call it subordinating the US national interest for personal, political gain. Either way, it qualifies as a “high crime or misdemeanour” defined as an impeachable offence under the constitution. Or, simpler still, as the House speaker, Nancy Pelosi, now suggests, you might call it plain old “bribery”, which the constitution explicitly lists alongside treason as grounds for a president’s removal from office.

However you slice it, the White House’s own account – not quite a transcript – of Trump’s phone call on 25 July reveals an abuse of power, one every bit as clear-cut as the abuses which saw Richard Nixon forced out in 1974. (He even added to the charge sheet on Friday, intimidating a witness via tweet, at the very moment she was testifying that she had felt threatened by him.)And, just like Watergate, there was a cover-up, as Trump aides hastily hid away that “transcript” on a top-secret server, where it would have lain undiscovered had it not been for a whistleblower.

In the Nixon case, impeachment never even got to a vote of the full House let alone a trial in the senate: a delegation of senior Republicans told Nixon the game was up, and he preferred to resign than be removed. It’s conceivable the Trump story will end the same way, and yet few expect it. The settled view in Washington – which has been wrong before – is that this will break on partisan lines: the Democratic-led House will vote to impeach Trump, but two-thirds of the Republican-dominated Senate will not be ready to convict him, as the rules demand, and he’ll be acquitted.

Why? It’s not because of the quality of the arguments Republicans are marshalling in the president’s defence. They dismissed Wednesday’s witnesses – a pair of devoted, non-partisan diplomats, as offering mere hearsay – they were like “a bunch of gossip girls”, said the Trump adviser Kellyanne Conway – because they had not personally discussed Ukraine with Trump. To which the obvious reply is: fine, let Trump’s closest aides testify instead of barring them from appearing, as the White House has done.

Others say that there’s no crime because eventually the US aid to Ukraine was unblocked and Kyiv never did produce any dirt on Biden. Memo to those Republicans: if you set a barn on fire and the barn survives, it’s still arson. Besides, the aid to Ukraine was released only once that unnamed government employee blew the whistle. It’s not much of a defence to say that you halted your criminal scheming the second you got caught. Nor will it do to say that Trump was set up by – guess who – George Soros, despite Republican efforts to point the finger in that direction.

“As the House speaker, Nancy Pelosi, now suggests, you might call it plain old “bribery”.’ Photograph: Jim Lo Scalzo/EPA

No, it’s not the ingenuity of Trump’s Republican advocates that is saving him. Nor is it their loyalty to him, even though Trump knows his base would forgive him almost anything – even a Fifth Avenue shooting – and congressional Republicans know that too, which is why they fear upsetting the faithful. Nixon’s devotees were also passionate, prompting Republicans on Capitol Hill to stick with him far longer than is usually remembered. What flipped many of them were the notorious Nixon tapes, whose transcripts revealed the president to be, as one scholar puts it, “profane, crude, and cynical”. That proved too much for many Republicans, who abandoned Nixon. No danger of that now: Republicans have always known that Trump is profane, crude and cynical – but they’ve backed him anyway.

Indeed, there is a terrible jadedness towards Trump, manifested in the press notices for Wednesday’s hearings which declared them worthy but lacking pizazz. The “shock” threshold has now been raised so high that, unless he commits a fresh horror more spectacular than any previous, the impulse is to let Trump off with a shrug. That’s not only the media’s fault. “The responsibility lies with us all for letting this lawless, kleptocratic presidency become normalised,” wrote the New York Times columnist Michelle Goldberg.

Still, the difference between 1974 and 2019 that matters most is in the political and media landscape. Partisan allegiances are now so entrenched that ever fewer Americans are able to see fault in their own side. When the evidence against Nixon was in, it took three TV networks and a half-dozen major newspapers to declare it damning and that was that: all but a fringe of society agreed on the basic facts. Now there is no such shared “fact base”. Fox News viewers are shown an alternative reality in which Biden is the villain and Trump the valiant scourge of corruption. If Nixon had had Fox News in his corner, Watergate would not have been “Watergate”.

The consequence is impunity. Note that Trump made that fateful call to Kyiv the day after Robert Mueller had testified on Capitol Hill, allowing Trump to believe he was off the hook on the Russia affair. He felt himself beyond the reach of the law. And this is the danger.

The former UK foreign secretary David Miliband warned that we are living in an “age of impunity”, in which states can murder and maim without consequence: think Assad and Syria, or Mohammed bin Salman and Jamal Khashoggi. There is a domestic analogue to that impunity and Trump embodies it. If he is allowed to get away with his crimes, the conclusion his successors will draw is that a US president is, in practice, above the law. Leaders around the world who look to the US will learn a similar lesson, that in the right circumstances you can get away with anything. That includes murder on Fifth Avenue – and much worse.

• Jonathan Freedland is a Guardian columnist