Let’s set aside, for the moment, the duty of lawmakers to the Republic and the Constitution and instead consider simple political self-interest. Even by this standard, Republicans’ behavior is beginning to look like masochism if not simple madness. When members of Mr. Trump’s party pooh-pooh his thuggish rantings and otherwise signal that they will overlook even his most dangerous behavior, they are inviting him to act out even more. Like a willful toddler, Mr. Trump lives to test limits.

Republican lawmakers need not attack Mr. Trump in order to stop enabling his worst impulses and begin distancing themselves from his corruption. They simply need to stop cowering. An obvious first step is for Congress to pass legislation protecting Robert Mueller’ s Russia inquiry, which has become the bane of Mr. Trump’s existence. The president has toyed with the idea of firing Mr. Mueller and his superior, Deputy Attorney General Rod Rosenstein, moves that would ignite a constitutional crisis. Lawmakers are deluding themselves to think that he won’t consider such radical acts again as his predicament grows more dire.

Much of the groundwork for a bill to protect the Russia investigation has already been laid, with a bipartisan plan having passed the Senate Judiciary Committee. Shamefully, Senator Mitch McConnell, the Republican majority leader, has refused to bring the bill up for a vote, insisting that it is unnecessary because of course the president would never fire Mr. Mueller. Mr. Ryan has spouted similar assurances. Then again, Mr. Ryan also laughed off the idea that Mr. Trump would strip his political critics of their security clearances, so clearly Republican leaders are not the best barometers of this president’s thinking.

Speaking of Mr. Ryan, the speaker needs to shut down the attacks on Mr. Rosenstein by Mr. Trump’s lackeys in the arch-conservative Freedom Caucus. Earlier this summer, Representatives Mark Meadows and Jim Jordan threatened to force an impeachment vote on Mr. Rosenstein, claiming that he was impeding Congress’s harassment — uh, “investigation” — of the Justice Department and the F.B.I. When that plan flopped, the men set their sights on holding Mr. Rosenstein in contempt of Congress — which doesn’t sound as dramatic, but would, if successful, provide Mr. Trump an excuse to oust Mr. Rosenstein and replace him with a lap dog.

Congress also needs to open investigations into the campaign finance violations to which Mr. Cohen pleaded guilty and said Mr. Trump directed — illegally using corporate funds to influence the election by funneling money to the women through The National Enquirer and Mr. Trump’s company .