Greg Gianforte celebrates his win. (Rachel Leathe/Associated Press)

“GIANFORTE GRABBED Jacobs by the neck with both hands and slammed him into the ground behind him.” That was Fox News reporter Alicia Acuna’s account of how Greg Gianforte allegedly assaulted Guardian reporter Ben Jacobs, the day before Mr. Gianforte won a special election to fill Montana’s at-large congressional seat. Along with an audio recording of the incident, the eyewitness accounts confirm that the now-congressman-elect engaged in brutish behavior. That he subsequently tried to blame Mr. Jacobs for the incident, in which the reporter was merely asking an honest question, makes Mr. Gianforte’s actions all the more inexcusable.

Inexcusable means inexcusable. The House, led by Speaker Paul D. Ryan (R-Wis.), who said “there’s never a call for physical altercation,” should have made clear that Mr. Gianforte would not be welcome in the chamber. Instead, the speaker said , “If he wins, he has been chosen by the people of Montana.” Other Republicans were even more forgiving. “It’s not appropriate behavior,” Rep. Duncan D. Hunter (R-Calif.) reportedly told the Associated Press’s Mary Clare Jalonick. “Unless the reporter deserved it.” According to Texas Tribune reporter Patrick Svitek, Texas Gov. Greg Abbott (R) held up a bullet-riddled gun-range target sheet and said, “I’m gonna carry this around in case I see any reporters.”

If Mr. Gianforte is convicted of his pending assault charge, he should resign. If he declines to step away, the House of Representatives should expel him. Otherwise, a man convicted of assaulting a reporter and who subsequently lied about the incident, will serve in a body whose very existence proclaims that even the most contentious disputes can be managed through debate and peaceful disagreement. At the least, Mr. Gianforte should be persona non grata on Capitol Hill, frozen out of committee assignments, treated with disdain.

Admittedly, expulsion would be unusual. Though the Constitution allows two-thirds of the House to expel any member for practically any reason, only five have been expelled: three for disloyalty during the Civil War period, and two for corrupt activities while in office. Mr. Gianforte can claim a measure of democratic legitimacy because the beating happened (barely) before the election.

It’s true that most Montana voters sent in their ballots early. But even if voters had chosen him with full knowledge of his behavior, lawmakers would have an obligation to make clear that speech is not to be met with violence, that the worst attack a questioner ever deserves is a tongue-lashing, that those chosen to conduct the nation’s business must meet at least a minimum standard of decorum.

Conservatives react with outrage at stories of intolerant college activists violently protesting right-wing speakers. Members of the Trump administration talk of fighting a war for “civilization,” a term that, if it means anything, stands for the proposition that disagreements are not settled brutally. Republicans must bring the same sense of moral certainty to the case of Mr. Gianforte — or they must admit they care about these essential principles only when it is politically convenient.