In 231 years, only three US presidents have been impeached. Now Donald Trump faces two courts. The case against him is simple, and established not only by officials speaking under oath, but by his own words and actions. It is that he attempted to pressure Ukraine’s government into interfering in a US election, withholding military aid. His administration attempted to cover matters up. He refused to cooperate with a constitutionally authorised congressional inquiry.

Yet the first court is almost guaranteed to acquit. The wrangling over the terms for the president’s trial in the Senate continues, but the forum is so rigged that it is vanishingly unlikely to convict him and end his presidency. It is not simply the Republican majority in the Senate that is to blame – a two-thirds majority is needed for conviction – but the way that Republicans cling to a man many of them despise. The jury’s foreman, Senate majority leader Mitch McConnell, has proudly stated that he will take his cues from the accused’s lawyers. Lindsey Graham, who with other senators must swear to deliver impartial justice, blithely announced: “I’m not trying to pretend to be a fair juror here.”

The second court is that of public opinion. While overall polls suggest slightly more people now support impeachment than oppose it, the gap is small and may be closing again. Broadly speaking, Democrats continue to support his trial; Republicans oppose it; independents are divided. In a CNN poll, more people thought the charges would help Mr Trump than harm him.

In part, this is because Mr Trump has lowered the bar for a president’s behaviour so sharply and so persistently. It is also because he and his supporters have portrayed this process as a partisan witch-hunt, rather than an essential part of the nation’s democratic protections. When supporters accuse the Democrats of trying to silence the 63 million people who voted for Mr Trump, it is satisfying but unhelpful to retort that they ignore the more than 65 million who voted for Hillary Clinton. This process must be for all the electors of 2016 and next year. A small but helpful step would be to include Justin Amash – the former Republican who has backed impeachment – in the Democrat trial team.

But only the Republicans can ensure a suitable hearing. They complain that the House saw only hostile witnesses and ludicrously describe a trial less fair than that of Jesus before Pontius Pilate. Meanwhile it is the White House that has blocked the appearance of Trump-appointed officials and the introduction of documents that would supposedly exonerate him. A large majority of voters – including a clear majority of Republicans – believe that Mr Trump should allow his chief of staff, Mick Mulvaney, and his former national security adviser, John Bolton, to testify. They are right. But they are likely to be disappointed.

Mr Trump has prospered by knowing no shame. But there is no question that he feels the sting of this case. Read his bizarre six-page letter to House Speaker Nancy Pelosi. Or recall his remarks on Barack Obama five years ago; were he impeached, Mr Trump said, “It would be a horror show for him … It would go down on his record permanently.”

The public’s immediate verdict on this case, and the one it returns in November 2020, may dismay all those who care about American democracy. The risk of impeachment backfiring is real, as Democrats have always known. But the danger of not acting was certain. It would send the worst message to Mr Trump and to the presidents who follow. History’s judgment in this case would not be kind.