In a floor speech the morning after the impeachment vote, Mr. McConnell ranted at length about the flawed process in the House’s investigation, claimed the articles were “constitutionally incoherent,” and he proclaimed it the Senate’s “duty” to end what he called an assault on norms and precedent.

As for the trial, Mr. McConnell aims to keep things perfunctory. He is disinclined to allow witnesses; he has expressed a willingness to ram through rules on a party-line vote; and — to repeat — he has already announced what the ultimate finding must be.

It may also strike some observers as rich that Mr. McConnell is suddenly concerned about process, precedent and norms. This is the same Mitch McConnell who held open a Supreme Court seat for almost a year to deny President Barack Obama his rightful pick; who repeatedly tweaked the Senate’s rules to grease the skids for Mr. Trump’s judicial appointees; who keeps a chokehold on the Senate floor — stymieing debate, frustrating members of both parties and reveling in his reputation as the legislative “grim reaper.”

Mr. Trump is correct that impeachment has exposed congressional hypocrisy. He’s simply confused as to its wellspring.

Considering how slippery Mr. McConnell can be, it may be that flamboyantly lashing himself to Mr. Trump’s defense team is a handy bit of hide-covering. Having put on such a grand display of fealty, the majority leader has plausible deniability in the event that some aspect of the proceedings winds up displeasing the White House.

But Mr. McConnell’s pledge to place the Senate at the president’s service puts other Republicans in an awkward spot — at least those still interested in maintaining a modicum of independence or integrity. And this week, a thin crack in conference unity appeared.

In an interview that aired on Christmas Eve, Senator Lisa Murkowski of Alaska told an Anchorage TV station she was “disturbed” by Mr. McConnell’s pledge to coordinate with the president’s legal team. It would be wrong to “prejudge” this matter, she said. “To me, it means that we have to take that step back from being hand in glove with the defense.” Mr. McConnell, she lamented, had “further confused the process.”