President Trump and his Republican enablers in Congress incited a violent attack Wednesday against the government they lead and the nation they profess to love. This cannot be allowed to stand.

Mr. Trump’s seditious rhetoric prompted a mob of thousands of people to storm the U.S. Capitol building, some breaking onto the House and Senate floors, where the nation’s elected representatives had gathered to perform their constitutional duty of counting electoral votes and confirming the election of Joe Biden as president.

It is fitting that some carried the Confederate flag as they attacked the seat of American government and forced the suspension of congressional debate. They shattered windows and broke doors, clashing with overwhelmed security forces as they shouted their support for Mr. Trump and their defiance of the lawful results of the 2020 election. One woman was killed. The nation’s leaders were sent scurrying for shelter.

Explosives were found in the Capitol and multiple locations around Washington. Pro-Trump protests also shut down statehouses around the country.

Mr. Trump sparked these assaults. He has railed for months against the verdict rendered by voters in November. He summoned his supporters to gather in Washington on this day, and encouraged them to march on the Capitol. He told them that the election was being stolen. He told them to fight. He told them he might join them and, even as they stormed the building, he declined for long hours to tell them to stop, to condemn their actions, to raise a finger in defense of the Constitution that he swore to preserve and protect. When he finally spoke, late in the day, he affirmed the protesters’ anger, telling them again that the election was stolen, but asking them to go home anyway. It was the performance of a man unwilling to fulfill his duties as president or to confront the consequences of his own behavior.

The president needs to be held accountable — through impeachment proceedings or criminal prosecution — and the same goes for his supporters who carried out the violence. In time, there should be an investigation of the failure of the Capitol Police to prepare for an attack that was announced and planned in public.

This is not just an attack on the results of the 2020 election. It is a precedent — a permission slip for similar opposition to the outcomes of future elections. It must be clearly rejected, and placed beyond the pale of permissible conduct.

The leaders of the Republican Party also bear a measure of responsibility for the attack on the Capitol.

Many in the G.O.P. have participated in the vigorous retailing of lies about the election. They have sought to undermine public confidence in democracy, questioning the legitimacy of Mr. Biden’s victory without providing any evidence for their claims. Their statements led some of those who trust them to conclude violence was necessary.

Few have been as explicit as Rudy Giuliani, the president’s personal lawyer, who earlier Wednesday suggested, “Let’s have trial by combat!” But even as extremists boiled up around the Capitol, lapping against the security barriers, Republicans in the House and the Senate were chipping away at democracy from the inside.

Senator Ted Cruz, Republican of Texas, invoked the 1877 commission that resolved the disputed presidential election of 1876 as a model for what he described as addressing reasonable doubts about the 2020 election. There is no factual basis for such doubts about the 2020 vote, but Mr. Cruz’s choice of analogy is historically resonant. In the 1876 election, white Democrats used widespread political violence to prevent Black people from voting and then demanded the end of Reconstruction as the price of the survival of a compromised Republic — ushering in an era of racial terror and cementing the exclusion of Southern Blacks from participatory democracy.

The modern Republican Party, in its systematic efforts to suppress voting, and its refusal to acknowledge the legitimacy of elections that it loses, is similarly seeking to maintain its political power on the basis of disenfranchisement. Wednesday’s insurrection is evidence of an alarming willingness to pursue that goal with violence.

It is clear that some Republican leaders are starting to fear the consequences of enabling Mr. Trump. Before the attack started, Senator Mitch McConnell, the majority leader, decried efforts by his fellow Republicans to overturn the results of the election. But his eloquence was the very definition of a gesture both too little and too late. They who sow the wind, reap the whirlwind.

Other politicians have had firmer convictions. Former presidential candidate Mitt Romney tweeted, “What happened at the U.S. Capitol today was an insurrection, incited by the President of the United States.”

The Constitution requires Congress to count and announce the results of a presidential election on Jan. 6 of the following year. While the mob was able to put that process on hold, it will not be able to prevent it, or Mr. Biden’s inauguration in two weeks.

But the attack is a reminder of the fragility of self-government.

Jan. 6, 2021, will go down as a dark day. The question is whether, even as Mr. Trump’s time in office ends, America is at the beginning of a descent into an even darker and more divided epoch or the end of one. The danger is real, but the answer is not foreordained. Republican politicians have the power, and the responsibility, to chart a different course by ending their rhetorical assaults on American democracy and rising in defense of the nation they swore oaths to serve.

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