And several advantages would accrue to the Democrats if Mr. Bolton were to testify. First, this would undermine the White House’s and Republicans’ plan to terminate the trial with a vote to acquit Mr. Trump by the end of this week. Second, Mr. Bolton could smash the White House’s and Republicans’ argument that there is no direct evidence that Mr. Trump himself directly linked military aid to Ukraine to a request that the Ukrainian president, Volodymyr Zelensky, announce an investigation of the Bidens. (Mr. Bolton reportedly said as much in an unpublished manuscript of his forthcoming book, causing the White House to threaten to block its publication.)

Third, the longer the trial drags on the greater opportunity there is for more damaging information to arrive on the Democrats’ doorstep. Even if gathering the two-thirds Senate vote to remove Trump from office remains beyond reach, a secondary goal of the Democrats is to make Trump’s acquittal as unglorious as possible. The more an acquittal hinges on preposterous limits on disclosure of information and on preventing witness testimony, the less valid it will seem.