President Trump and special counsel Robert S. Mueller III. (Jabin Botsford; Win McNamee/The Washington Post; Getty Images)

FEARS THAT President Trump will upend the Justice Department — by firing special counsel Robert S. Mueller III, Attorney General Jeff Sessions, Deputy Attorney General Rod J. Rosenstein or some combination of the above — have never been so warranted. The New York Times reported Tuesday that White House discussions of a purge have been more serious than previously known. White House press secretary Sarah Huckabee Sanders insisted Tuesday that the president believes he has the power to fire Mr. Mueller. And Mr. Trump reportedly reached new heights of rage after federal prosecutors obtained a lawful search warrant against Michael Cohen, his personal lawyer. “Many people have said, ‘You should fire him,’ ” the president said about Mr. Mueller on Monday. Mercurial, undisciplined and facing few internal checks, the president may lash out.

If Congress intends to check Mr. Trump, now is the time to do it. So far Republican lawmakers have offered only words. John Cornyn (R-Tex.), the Senate’s No. 2, said Tuesday that firing Mr. Mueller would be a mistake. Senate Judiciary Committee Chairman Charles E. Grassley (R-Iowa) was more forceful, saying it would be “suicide for the president to want, to talk about firing Mueller,” echoing Sen. Lindsey O. Graham’s (R-S.C.) insistence last month that canning the special counsel “would be the beginning of the end of his presidency, because we’re a rule-of-law nation.”

While better than silence or acquiescence, these warnings fail to communicate that the president would face a serious institutional response for igniting a national crisis in a fit of pique. They also fail to convey that firing Mr. Rosenstein or Mr. Sessions to foil Mr. Mueller’s probe would elicit a strong backlash, too. This is particularly true when Mr. Trump can turn on Fox News to see Rep. Devin Nunes (R-Calif.), a Trump toady who is inconceivably still the chairman of the House Intelligence Committee, suggesting, also inconceivably, that the House might impeach Mr. Rosenstein and FBI Director Christopher A. Wray.

Lawmakers could send a message that, as a body, Congress can and will draw red lines, despite whatever Mr. Nunes says. That starts by passing a bill protecting Mr. Mueller from the president’s wrath. Sponsors have merged two competing versions of this legislation into a single proposal, which would allow a special counsel, in this case Mr. Mueller, 10 days to seek judicial review of his firing.

If the bill became law, it still would not prevent the president from replacing the deputy attorney general overseeing the special counsel probe, which would leave an avenue open for Mr. Trump to tamper with the investigation. In fact, the bill is not even likely to become law, because the president would almost certainly veto it. Yet its passage would nevertheless signal Congress’s determination to respond to egregious White House meddling in an investigation of the president and his circle. It could help deter Mr. Trump from making a rash move. The only reason not to pass the bill is to protect GOP lawmakers from taking a tough vote “for” or “against” the president, which is no good reason at all.

Passing the legislation would be only a start. Lawmakers should fashion a plan, now, for the day after the president begins handing out pink slips. If such a plan exists, the country has yet to see it.