The impeachment inquiry into Donald Trump came gradually, then suddenly. Though this is a process so rare that only three predecessors have faced it, calls for his impeachment began before he had even taken the oath of office. They drew into view more clearly when the Democrats seized the House. Until this week, divisions held the caucus back. Then it swung behind the demand with stunning speed. Speaker Nancy Pelosi, who had warned that the process was “not worth it”, succumbed to the inevitable, announcing an inquiry into allegations that the president pressured a foreign country to harm a political rival.

Impeachment is both an issue of justice and accountability, and a political calculation. No one expects Mr Trump to be convicted by the Senate if impeached by the House; Republicans have repeatedly shown their determination to stand by him (though most Republicans stood by Richard Nixon until late in the day). What shifted the Democrats was the emergence of a whistleblower complaint deemed credible and urgent by the inspector general. Such complaints must be reported to Congress, but the acting director of national intelligence refused to pass it on. It has now, at last, been delivered.

The release of a memo of Mr Trump’s call to the Ukrainian president – with whom he held an awkward meeting in New York on Wednesday – puts meat on those bare bones. He asks for a “favour”. He urges Volodymyr Zelenskiy to work with the US attorney general and his personal lawyer Rudy Giuliani to investigate Joe Biden, the former vice-president and Democratic presidential candidate, and his son (there is no evidence of wrongdoing on the Bidens’ part). We know that he withheld military aid to Ukraine one week before that call, and has given contradictory explanations – including suggesting that there would be nothing wrong with him withholding funding to a country if it refused to root out “corruption”.

Unlike his campaign’s dealings with Russia, this is a simple, easily understandable issue directly involving Mr Trump himself. It took place during his presidency. The withholding of the complaint is an issue in itself, regardless of its contents. The inquiry will force Republicans to choose between backing him again – potentially alienating moderates – or angering his base.

But the 1998 proceedings against Bill Clinton backfired politically, and Mr Trump loves a fight. The inquiry already shows signs of firing up his supporters: a fundraising drive for the “Official Impeachment Defense Task Force” raised a million dollars for the Republican National Committee and Trump campaign on Tuesday. And it risks attention focusing on the behaviour of Mr Biden and his family, and the claim that the Democrats are part of the Washington “swamp”, rather than upon what Democratic presidential candidates can offer voters.

While inquiries can go in unpredictable and important new directions, there is also a danger of politicians and the media becoming caught up in a hunt for new and worse transgressions, allowing Mr Trump to brush aside what we already know – and we know plenty. What we know of his discussions with Mr Zelenskiy is shocking, but entirely in line with his blatant refusal to distinguish between his office and his political and personal interests.

Impeachment is a last resort – the most serious action that Congress can take against a president – but necessary given Mr Trump’s refusal to abide by political norms. Democrats have unified around a purpose: what are they in Congress for, if not to hold the president to account? The power of accountability does not grow weaker with use, but stronger. To act now carries risks. But to stand by would be far worse, sending a message of impunity to Mr Trump, and those who succeed him. Instead they have told him, and America, that no one is above the law.