PRESIDENT TRUMP became on Wednesday the first president to be impeached twice, as 222 Democrats and 10 Republicans in the House voted to indict him for his incitement of the Capitol riot. Among them was Rep. Liz Cheney (Wyo.), the third-ranking Republican in the House GOP caucus, who, in following her conscience, bravely exposed herself to severe criticism from the pro-Trump elements of her party.

Reports indicate some Senate Republicans, including Majority Leader Mitch McConnell (Ky.), are considering voting to convict Mr. Trump in a future Senate trial. Others who shy from impeachment are considering censure or trying to force Mr. Trump to resign. After four years of outrage and humiliation, at least some key GOP leaders are breaking with the worst president in modern U.S. history.

There is no doubt Mr. Trump deserved to be impeached. As Ms. Cheney explained, “The President of the United States summoned this mob, assembled the mob, and lit the flame of this attack. Everything that followed was his doing. None of this would have happened without the President. The President could have immediately and forcefully intervened to stop the violence. He did not.”

Impeaching Mr. Trump now does not obviate the need for further investigation. Large questions remain: How close did the mob come to harming members of Congress or the vice president? Was there a conspiracy? Were other GOP leaders involved? What was the source of the pipe bombs discovered on Jan. 6? Why was the Capitol so exposed and backup seemingly so unavailable? What measures must be taken to ensure this never happens again?

Even so, though “much more will become clear in coming days and weeks,” Ms. Cheney noted, “what we know now is enough.” The known scale of Mr. Trump’s offense already qualifies him for severe punishment, and the danger he poses to American civic life is too substantial to ignore. The impeachment power must be nimble enough to address a situation like Watergate, where there was time to conduct a deep investigation to ascertain the president’s guilt, and the Capitol raid, in which the president’s obvious culpability and the continuing risk he poses warrant summary removal.

That is why we hope Mr. McConnell reconsiders his reported reluctance to move swiftly toward a trial. The president’s term ends in any case next Wednesday, but removal first would send an important message. It also would be unquestionably legitimate, whereas some scholars believe the Senate does not have the constitutional right to try, convict and bar from future office a president after his term ends. A delayed or a lengthy Senate trial could overshadow President-elect Joe Biden’s opening days, slowing confirmation of his appointees — with attendant risk to national and homeland security — and derailing his legislative agenda.

If Mr. McConnell refuses to convene the Senate this week, senators must move with dispatch once they have convened, and split their time between trying Mr. Trump and enabling the launch of the Biden administration. But the nation would be better served by a prompt trial ending in the guilty verdict Mr. Trump deserves.

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