George T. Conway III is Trump’s most eloquent critic. If you follow him on Twitter, you will know that he has been consistent in calling out Trump for his outrageous behavior and his illegal actions. (He is the husband of Kellyanne Conway, Trump’s senior advisor, and some speculate that she is Anonymous, the inside whistleblower.)

He writes today in the Washington Post that Republicans have lashed themselves to an unhinged and unpredictable figure, who will pull them down too.

Members of Congress should be aware of the famous words of lawyer Joseph Welch, who represented the U.S. Department of the Army, defending it against charges of Communism by Senator Joseph McCarthy. Confronting a sneering, badgering Senator McCarthy, who was accusing one of his junior associates of Communist sympathies, Mr. Welch said, “At long last, have you left no sense of decency?” Welch helped to end the infamous career of McCarthy, whose like has not been seen again until now, in the language and behavior of the current president of the United States, whose first resort to any challenger is ridicule and contempt.

George T. Conway III is a lawyer in New York and an adviser to the Lincoln Project, an anti-Trump super PAC.

In his unhinged letter Tuesday, President Trump accused House Speaker Nancy Pelosi of having “cheapened the importance of the very ugly word, impeachment!” A few days earlier, he accused Democrats of “trivializing impeachment.” 

If anything has cheapened or trivialized the process by which Trump was impeached, it was House Republicans’ refusal to treat the proceedings with the seriousness the Constitution demands. Unable to defend the president’s conduct on the merits, GOP members of the House resorted to deception, distortion and deflection: pretending that Trump didn’t ask President Volodymyr Zelensky of Ukraine to investigate Trump’s political rival; claiming that Ukraine interfered with the 2016 election; and throwing up all manner of silly assertions of procedural unfairness. 

Now, as the process moves to the Senate, Republican senators threaten the ultimate cheapening and trivialization of Congress’s constitutional obligations: holding a “trial” that would be nothing but a sham. 

Specifically, they threaten to conduct a trial without witnesses, which wouldn’t be a trial at all. In civil or criminal litigation in a jury case, the only way for a defendant to avoid a trial is for a judge to rule that there was no evidence from which the jury could find for the other side. However much the Republicans may pretend otherwise, that isn’t the case here — the president isn’t entitled to summary judgment. The evidence against him is far too strong. No doubt that is the real reason they wish to avoid a full-blown trial. 

In addition, judicial trials traditionally give parties the right to subpoena all relevant witnesses, even witnesses who haven’t given evidence before. Indeed, the Senate’s standing impeachment rules provide for the issuance of subpoenas at the request of either side. Even at the impeachment trial of President Bill Clinton, which involved much simpler facts, and where a complete evidentiary record had been developed by an independent counsel before a federal grand jury that heard the testimony of all witnesses, including Clinton, the Senate allowed videotaped deposition testimony to be taken and introduced into evidence. 

In any event, the fact that senators swear an oath to “bear true faith” to the Constitution, and the fact that the Constitution requires the Senate to “try all Impeachments,” should require them to hold a real trial with live witnesses. But if that isn’t enough to persuade them, Republican senators should consider at least two significant practicalities. 

The first is that the Ukraine investigation is only three months old. Senate Majority Leader Mitch McConnell (R-Ky.) has cited this as evidence that the impeachment was rushed and unsupported. Actually, the opposite is true: A remarkably strong case was assembled in an unusually short time, even in the face of extraordinary obstructionism from the administration, which directed numerous witnesses not to testify. The Watergate investigation, by contrast, spanned roughly two years; the Starr investigation and Clinton impeachment proceedings, 11 months. 

What that tells us is that plenty more evidence remains to be unearthed. We know already of the witnesses whom Senate Minority Leader Charles E. Schumer (D-N.Y.) proposes should testify, but whose testimony was blocked by Trump: acting White House chief of staff Mick Mulvaney; Robert Blair, a senior adviser to Mulvaney; former national security adviser John Bolton; and Michael Duffey, a top official at the Office of Management and Budget. No doubt there are volumes of electronic and physical documents that remain to be produced. 

One way or another, the gist of that evidence will seep out, as truth inevitably does. The president’s personal lawyer, Rudolph W. Giuliani, will continue to make admissions, and, if he were to be criminally prosecuted, evidence would come out in his prosecution. Others — most notably Bolton — will write books. There will be more leaks to intrepid journalists. Perhaps more whistleblowers will step forward. 

Meanwhile, the House can still investigate, if it so chooses. In fact, it should. After all, not only does the House have a continuing obligation of oversight, but also there is no double-jeopardy prohibition on impeachment: If more damning evidence surfaces, there is no constitutional reason Trump couldn’t be impeached again. 

So common sense should tell senators that, even if Trump is acquitted in a short-circuited trial, that won’t prevent the evidence from revealing whether an acquittal was a just one. If Republican senators cut the trial short, they run the risk of being refuted and shamed on the pages of history by the very evidence they sought to suppress. 

But that’s not the only practical reason they shouldn’t cut the trial short. A second reason — perhaps the ultimate reason — is President Trump. 

For the extraordinary evidence of the Ukraine scandal isn’t a one-off. Putting his interests above the nation’s is what Trump instinctively does.

Trump’s written tirade to Pelosi confirms the point: It shows that, even as he is being impeached, he still has no idea why — and thus no idea what his presidential duties require. He hasn’t learned his lesson, and never will.

And that is the ultimate point Republican senators who care about their legacies should consider: They run the risk of being refuted and shamed on the pages of history not just by the evidence — but by Trump himself.