While much is unpredictable about the attempt to impeach Donald Trump, one thing can be anticipated with certainty. The US president will show no respect for the process – and no contrition if found guilty.

Mr Trump’s approach will be consistent with his already familiar political style: deceit, cronyism, distraction and bullying. It is the success of that technique that makes impeachment necessary and also difficult. A president who is so obviously unworthy of the office must be held accountable and yet, because Mr Trump’s methods have corrupted American public discourse, the unworthiness is not at all obvious to a large swathe of voters.

Democrats have their work cut out persuading many US citizens that there is even a case to answer. They must overcome a conservative propaganda machine that presents impeachment as a crooked enterprise in itself. Facts that should be beyond dispute battle for attention with an army of lies. Partly to overcome that challenge, Democrats have kept the draft articles of impeachment, published this week, succinct. They set out a streamlined version of Mr Trump’s offences, focusing on abuse of power and obstruction of Congress. At the heart of the matter is the alleged attempt to use US military aid to induce Ukraine’s president, Volodymyr Zelenskiy, to collude in discrediting Joe Biden, the former US vice-president. Mr Trump denies any such scheme.

The longer trail of misdeeds dating back to the 2016 election campaign is not explicitly cited in the articles. Many Democratic supporters would have preferred a wider-ranging account of corruption, racism, deception, interference with the judiciary and reckless dereliction of duty. The defence of a narrower focus is both legal and political. The charges have to stick. Democrats in Congress felt that the Ukraine affair contained the vital ingredients of judicable “crime and misdemeanour” – the constitutional threshold for impeachment. Nancy Pelosi, the House speaker, also had to contend with qualms from colleagues who feel constrained by undecided voters who are not viscerally hostile to Mr Trump. Much though it would be gratifying to scoop up everything appalling about the current White House regime and land it on the president with a cathartic knockout blow, that is not available in the climate of US politics. A perception of indiscriminate assault on the commander-in-chief could backfire on Democrats who, for now, just about have the balance of public opinion on their side.

Whether the president deserves to be removed from office should not be decided by opinion polls. But next November’s ballot creates a febrile environment in which gravitation towards polarised positions is more powerful than any bipartisan instinct. To complete an impeachment will require a two-thirds Senate majority, which depends on Republican votes. Those senators must either believe that their president is so toxic that electoral interest requires abandoning him, or they must value the resilience of the constitutional order above popularity with the voters who propelled Mr Trump to office. Neither seems likely. If there were a limit to what most Republicans can tolerate in aberrant behaviour, the president would have found it by now. He has crossed every line that might have been drawn. He cannot clear a bar set at the lowest conceivable threshold of decency, competence or integrity.

But the impeachment process serves a function that goes beyond next year’s electoral tests. It asserts the supremacy of law in a political system imperilled by a leader who believes with despotic certainty in his own immunity from criticism or sanction. It is not a partisan move against the president but a defence of the foundational principles of the American republic. To declare Mr Trump unfit for office is to anchor US democracy in a self-evident truth when it is dangerously adrift on a sea of lies.