“One thing we know is you, Rosen, aren’t going to do anything to overturn the election.” These, according to a new Senate Judiciary Committee report, were the words of President Donald Trump, pressuring the acting attorney general, Jeffrey Rosen, to upend a free-and-fair vote. Three days after Mr. Trump uttered them, a mob he had inflamed with lies ransacked the Capitol as lawmakers met to count duly cast electoral votes.

The Senate report details how Mr. Trump tried persistently to enlist the Justice Department in his scheme to overturn the 2020 election results. His pressure campaign, after Attorney General William P. Barr resigned in December, featured calls and meetings with Mr. Rosen and other top Justice Department staff. It continued as Mr. Trump sent them a preposterous petition he wanted them to file with the Supreme Court asking the justices to void Joe Biden’s victory. It reached its zenith in a cockamamie plot to force Mr. Rosen to pressure state governments to cook the results or be replaced by Jeffrey Clark, a lower-ranking Justice official who would go along with the scheme.

Mr. Trump failed because Mr. Rosen and other officials in key positions refused to cooperate and threatened to resign. But they could not stop Mr. Trump from forcing the resignation of the U.S. attorney in Atlanta and replacing him with a lawyer the then-president thought would pursue the fraud investigations he wanted to see.

Senate Republicans played down these revelations, arguing that, following the Russia investigation, it was reasonable for Mr. Trump to mistrust the Justice Department and the FBI. But, leaving aside the fact that the Russia probe was a well-founded and legitimate counterintelligence investigation, Mr. Trump, in this case, was not exercising reasonable skepticism; he was trying to hold on to power against the wishes of the American people, based on widely debunked mistruths about the 2020 vote. Anyone seeking to play down that fact today is complicit in his plot to undermine U.S. democracy.

The seriousness of Mr. Trump’s effort to nullify an election, his continuing lies about the results and the willingness of so many Republicans to indulge those lies call for several responses.

The investigations must continue. The House’s Jan. 6 committee should compel Mr. Clark, who did not cooperate with the Senate Judiciary panel, to testify. The House and the Justice Department must enforce the committee’s subpoenas, which several Trump confidantes appear prepared to flout on the former president’s say-so. The National Archives should turn over documents immediately. If courts are involved, judges must act with urgency. Mr. Trump learned how to run out the clock by gaming the judicial system; courts must not let that happen again.

Most urgently, Congress must reinforce elements of the nation’s democratic infrastructure vulnerable to exploitation by bad actors such as Mr. Trump. It should revamp the ancient Electoral Count Act to limit partisan interference in presidential vote tallying, and it should impose federal election standards that insulate state election officials from political pressure. Republicans who still respect the Constitution should be willing to join in this effort.