To impeach a US president once is a grave enough business. To do it twice is unique in all history. And still the House of Representatives had no other choice on Wednesday. Donald Trump had neither resigned over the siege of Congress that he incited last week, nor unreservedly deplored it. His vice-president Mike Pence had not used the 25th amendment to oust him. A legislature that let him serve out his last week or so for the sake of convenience would have been derelict in its duty to the law.

Action against the president cannot stop at symbolic censure. Barring the emergence of evidence to the contrary, Republican senators should prepare to convict Mr Trump when the House refers the issue to them. That this will take courage — some in the party fear violent reprisals — is its own commentary on US civic health. The Capitol evokes a fortress at present, with perimeter fencing and thousands of armed guardsmen in fatigues. Airbnb has even cancelled bookings in Washington for the inauguration of president-elect Joe Biden next week. A supermajority for conviction seems improbable in these circumstances. To go by the gnomic hints from Mitch McConnell, though, the leader of Senate Republicans, it should not be ruled out.

If the vote reaches the Senate after Mr Trump’s departure, it is still worthwhile. The prize, his exclusion from public office, is as dear to some in his party as it is to any liberal. Nor should Democrats dread the loss of Mr Biden’s first weeks in power to this process. He has suggested that senators “bifurcate” their work, dealing with administration business and holding the Trump trial concurrently. None of this asks too much of a legislative chamber that flatters itself as the world’s grandest. It is hard to think of a more proper use of Senate time than uncovering and airing the details of what transpired on January 6.

At the very least, impeachment should make Mr Trump trim his behaviour in the time he has left. But even if he avoids conviction, he is still open to criminal prosecution for the storming of Congress and other deeds once he leaves office. Any action against him, political or judicial, entails a risk to public peace. Some of his core followers have shown the extent of their loyalty to him. Indeed, eminent Republicans cite the danger of an aggressive backlash as a case against impeachment.

It is not a frivolous objection. It is a reason to commend the 10 Republicans, such as Representative Liz Cheney, who voted for impeachment. But nor can a law-governed nation constantly second-guess the public reaction before applying the law. That way lies a dark precedent and a paralysed, fearful state. Had the Senate convicted and removed Mr Trump after his first impeachment a year ago, no doubt the outrage on the right would have been intense. But the storming of Congress, and the attendant loss of life, might never have happened. The past four years have been a lesson in the folly of postponing hard decisions and hoping for the best.

The trope that a week is a long time in politics takes on an ominous connotation now. In his remaining days as president, Mr Trump still has wide latitude in foreign policy. He remains commander-in-chief of the world’s mightiest armed forces. His potential to do harm on his way out has not been neutralised by his impeachment. What it has done, at the bare minimum, is transmit a moral signal to the world about what US democracy is willing to tolerate. It falls to the Senate to decide its exact strength.