Donald Trump’s iniquities need no rehearsing here. The US would be better off without him as president. His departure would be good news for the rest of the world. Even more importantly, his removal might be pivotal in a larger endeavour – the rebuilding of confidence in the world’s democracies. Nevertheless, it would be a mistake to impeach him.

Senior Republicans hesitate to criticise Trump after Manafort and Cohen verdicts Read more

This has been a devastating week for Trump. His personal lawyer Michael Cohen – think Tom Hagen to Trump’s Vito Corleone – has pleaded guilty to two campaign spending law violations that directly implicate Trump in authorising hush money payments. Meanwhile the president’s former campaign manager Paul Manafort has been convicted on eight bank and tax fraud charges arising out of the investigations led by the special counsel Robert Mueller. Each is an enormous blow. The Cohen plea, in particular, is further evidence, if it were needed, of Trump’s manifest unfitness for office. Predictably, there is talk of presidential impeachment in the air. It is easy to understand why. The desire to rid the state of a disgraceful and perhaps criminal leader is understandable and righteous. But impeachment is just about the most inappropriate and double-edged weapon for achieving this goal.

Let’s remember what impeachment actually involves. The House of Representatives must prepare and vote, by simple majority, for charges which are then the basis of a trial in front of the 100-member US Senate. A two-thirds vote in the Senate is required to convict and remove the president from office. If 34 senators vote not to impeach, the president stays.

Play Video 3:27 How to impeach a US president – video

In the current Congress, the Republicans have a 37-vote majority in the 435-seat House and a two-vote majority in the 100-seat Senate. Both bodies have become strikingly partisan in the past 30 years. Control of the House may change in the midterm elections on 6 November. That might allow impeachment to start. But Republican Senate numbers will not fall below the 34 necessary to block impeachment.

Impeachment is a political process. It is a trial by politicians. Party loyalties and re-election prospects matter as much as the facts or the charges. The only circumstances in which impeachment can rise above the inevitable taint of partisanship are if the facts and charges are incontestably threatening to the republic and if representatives from the president’s own party are prepared to vote against him for the perceived good of the nation.

These conditions were satisfied in Richard Nixon’s case in 1974. Nixon, a Republican, faced draft impeachment proceedings in the House over the White House’s Watergate cover-up. Early in August 1974, Republican support ebbed away after the release of tapes showing that Nixon had blocked the Watergate investigations and had lied about it. The facts now showed that the president had acted criminally; as a result, House and Senate Republicans were not prepared to defend him. Nixon resigned, and impeachment never took place.

But the same conditions were not satisfied in Bill Clinton’s case in 1998, which I witnessed at first hand in Washington. Here, everything was partisan from start to finish. Republicans had long craved the opportunity to bring Clinton down, while Democrats stood by him throughout. The criminal charges of perjury and obstruction of justice were far less weighty than those in Watergate, and the process was both polarised and polarising. Public opinion rallied behind Clinton and the charges were thrown out. Clinton left office two years later, with some of the highest approval ratings of any president.

Impeaching Trump would risk being more like the Clinton case than the Nixon one. In order for any attempt to impeach Trump to even get off the ground, the midterm elections are crucial. Many Democrats will be mobilised by the possibility of impeachment. But so will many Republicans. Polling suggests that Republican voters are more set in their opposition to impeaching Trump than Democrats are in support of impeaching him. It is possible that impeachment would be counter-productive on the doorstep for Democrats, which is why Democratic congressional leaders are not pushing it.

Even if Democrats do win the House in November, impeachment would be intensely divisive. It might even help rally the country behind the president. True, Trump does not possess Clinton’s ability to attract support across the spectrum. But nor, yet, is Trump accused of committing the level of offence – as president – that Nixon committed. The charges against Nixon related directly to way he conducted his presidency. Those against Trump apply to the period before he entered the White House.

In other words, there is no way, at this stage, that an impeachment move against Trump would succeed – because there will always be 34 senators who will vote for him to stay. Nor would it help to unify America around an alternative. Trump would still be president, and he would still be president of a more deeply polarised nation than ever. This would not be Watergate reborn.

Democrats may calculate that all this is a risk worth taking. Impeachment, or even the possibility of impeachment, might tie the administration up, limiting its options to take destructive new policy initiatives. They may hope, too, that an impeachment effort would mobilise the Democrats’ base and divide Republicans – a mirror image version of what happened in 2000, when George W Bush won an election that was otherwise ripe for the Democrats. But this assumes, wrongly, that what works on the right of politics also works on the left. A failed impeachment is more likely to embolden the president and strengthen Trump’s chances of winning re-election in 2020.

America can certainly be so much better than it is today. The reruns last week of Barack Obama’s tears when listening to Aretha Franklin were a heart-touching encapsulation of the moral dignity that America has thrown away in just two years of Trump. But there are very few short cuts in politics.

For progressive opposition parties, in Europe as in the US, successful democratic politics is still about winning the right to be heard, about finding leaders who can speak persuasively for credible change, and about winning an electoral argument based on hope, reason, inclusivity and civility. That is what Obama managed to do, and it is still the only sustainable way. In the aftermath of the financial crash, amid new extremes of inequality and with a rebirth of militant white nativism, it ought to be clear that what needs defeating is not just Trump, but Trumpism.

• Martin Kettle is a Guardian columnist