Schumer Whiffs Impeachment: The Democratic leader succeeds only in unifying the Republican caucus.

Mr. Schumer nonetheless had an opportunity to do what Democrats keep pretending to do: Take the proceedings seriously. Had he reached out to Majority Leader Mitch McConnell to work on an agreement—had he privately spoken of the need for the Senate to come together and publicly refrained from partisan jabs—he’d have had an audience. Republican senators—in particular those up for re-election or retiring—don’t want to be accused of bias. As of last weekend, enough GOP senators to matter were still on the fence about Democratic demands for more witnesses.

Not so much anymore. Mr. Schumer managed to annoy them, in particular with his first-day stunt of keeping the chamber in session until nearly 2 a.m., forcing vote after vote on witness motions that he knew would fail. Mr. McConnell’s organizing resolution already provided for these votes later in the process, making the Schumer motionathon nothing more than an inconvenient stunt.

He managed to offend Republicans too, by claiming they’ve already failed to “rise to their constitutional mandate,” are complicit in a “coverup” and don’t “want a fair trial.” He’s derided the president’s legal team and excoriated Republican witness proposals. He insulted his colleagues’ intelligence by arguing both that they need more evidence and that the case against Mr. Trump is already cut and dry. Reps. Adam Schiff and Jerry Nadler haven’t helped, with their presumptuous lectures on the senators’ obligations and duties.

Mostly, Mr. Schumer’s partisan accusations and threats have further exposed for Republicans his real goal with this trial: becoming Senate majority leader. Mr. Schumer knows the trial will end with acquittal. His nonstop attacks on Republican integrity are designed to squeeze vulnerable senators and help his party pick up seats in states like Maine or Colorado.