PRESIDENT TRUMP’S refusal to accept his election defeat and his relentless incitement of his supporters led Wednesday to the unthinkable: an assault on the U.S. Capitol by a violent mob that overwhelmed police and drove Congress from its chambers as it was debating the counting of electoral votes. Responsibility for this act of sedition lies squarely with the president, who has shown that his continued tenure in office poses a grave threat to U.S. democracy. He should be removed.

Mr. Trump encouraged the mob to gather on Wednesday, as Congress was set to convene, and to “be wild.” After repeating a panoply of absurd conspiracy theories about the election, he urged the crowd to march on the Capitol. “We’re going to walk down, and I’ll be there with you,” he said. “You’ll never take back our country with weakness. You have to show strength, and you have to be strong.” The president did not follow the mob, but instead passively watched it on television as its members tore down fences around the Capitol and overwhelmed police guarding the building. House members and senators were forced to flee. Shots were fired, and at least one person was struck and killed.

Rather than immediately denouncing the violence and calling on his supporters to stand down, Mr. Trump issued two mild tweets in which he called on them to “remain” or “stay” peaceful. Following appeals from senior Republicans, he finally released a video in which he asked people to go home, but doubled down on the lies fueling the vigilantes. “We love you. You’re very special,” he told his seditious posse. Later, he excused the riot, tweeting that “these are the things and events that happen when a sacred landslide election victory is so unceremoniously & viciously stripped away.”

Supporters of President Trump crossed barricades and began marching toward the back of the U.S. Capitol on Jan. 6. (The Washington Post)

The president is unfit to remain in office for the next 14 days. Every second he retains the vast powers of the presidency is a threat to public order and national security. Vice President Pence, who had to be whisked off the Senate floor for his own protection, should immediately gather the Cabinet to invoke the 25th Amendment, declaring that Mr. Trump is “unable to discharge the powers and duties of his office.” Congress, which would be required to ratify the action if Mr. Trump resisted, should do so. Mr. Pence should serve until President-elect Joe Biden is inaugurated on Jan. 20.

Failing that, senior Republicans must restrain the president. The insurrection came just as many top Republicans, including Senate Majority Leader Mitch McConnell (Ky.), were finally denouncing Mr. Trump’s antidemocratic campaign to overturn the election results. A depressing number of GOP legislators — such as Sen. Josh Hawley (Mo.), Sen. Ted Cruz (Tex.), House Minority Leader Kevin McCarthy (Calif.) and House Minority Whip Steve Scalise (La.) — were prepared to support Mr. Trump’s effort, fueling the rage of those the president has duped into believing the election was stolen.

Mr. McConnell, to his lasting credit, was not. “President Trump claims the election was stolen,” he said. But “nothing before us proves illegality anywhere near the massive scale, the massive scale, that would have tipped the entire election. . . . If this election were overturned by mere allegations from the losing side, our democracy would enter a death spiral.” He added: “I will not pretend such a vote would be a harmless protest gesture while relying on others to do the right thing.” As if to prove his point, the Trump mob would soon climb up the Capitol walls, and Mr. McConnell and his colleagues would seek refuge in secured locations.

Now that the stakes are viscerally clear, Mr. McConnell and every other Republican, almost all of whom bear some blame for what occurred on Wednesday, have an overriding responsibility to the nation: stopping Mr. Trump and restoring faith in democracy. That began Wednesday night with the resumption of the congressional session and the continuance of the electoral vote count. Some of the lawmakers who sought to benefit from Mr. Trump’s mob-stoking rage suspended their cynical posturing — though they will always bear the stigma of having contributed to the day’s shameful events.

The chaos confirmed once again the voters’ wisdom in rejecting Mr. Trump in favor of Joe Biden. The president-elect rose to the moment. “I call on this mob, now, to pull back and allow the work of democracy to go forward,” Mr. Biden said. “It’s not protest. It’s insurrection.” He concluded: “Today is a reminder, a painful one, that democracy is fragile.”

Mr. Biden is right. Rules, norms, laws, even the Constitution itself are worth something only if people believe in them. Americans put on their seat belts, follow traffic laws, pay taxes and vote because of faith in a system — and that faith makes it work. The highest voice in the land incited people to break that faith, not just in tweets, but by inciting them to action. Mr. Trump is a menace, and as long as he remains in the White House, the country will be in danger.

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