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.

As in the case of the Mueller report, this won’t stop Mr. Trump from declaring a fabulous victory. But the Democrats, with the Republicans in control of the Senate and so far essentially united on not convicting the president and removing him from office, have to go with what they can get. Democrats publicly brush off suggestions of a Biden-for-Bolton witness swap, but that could be a negotiating position.

It’s odd that after three years of a Trump administration, Republicans didn’t foresee the danger in putting their political careers in the hands of a man who’s out solely for himself and has a very distant relationship with the truth. But they clearly felt that they had no choice, and thus they enabled Mr. Trump to take them out on his limb. They are so frightened of him and his base that they have accepted his demand that his bullying phone call with Mr. Zelensky be considered “perfect.” This has left them, as well as him, no wiggle room. Which makes a Bolton appearance all the more dangerous.

Also, having Mr. Bolton testify offers him and his newfound Democratic allies an opportunity to make other damaging disclosures that may not yet have been discovered in his book or may not lie therein. He can elaborate on another bit of damaging goods from his book that has already been revealed: that in Bolton’s view (he’s not alone in this) Mr. Trump has seemed particularly complaisant toward autocrats. Representative Adam Schiff, chairman of the House Intelligence Committee and lead prosecutor in the impeachment trial, had planned to investigate whether Mr. Trump’s foreign policy has been guided by his business investments. Mr. Bolton may know if this occurred.

Mr. Trump’s company, from which he hasn’t divested his interest, has property in at least two countries led by autocrats with whom he’s on especially friendly terms: Turkey and the Philippines. Mr. Bolton has written that he worried about Mr. Trump’s particular warmth toward the leaders of China and Turkey. Might Mr. Trump’s subdued reaction to demonstrations in Hong Kong and to Turkey’s decision to invade northern Syria have anything to do with Trump company business?

Mr. Bolton may also know about other conversations with foreign leaders locked in the supersecret computer system where the full conversation between Mr. Trump and Mr. Zelensky was consigned. We’ve only seen a scrubbed transcript of that conversation. Why? And are there other conversations with foreign leaders revealing blunders or abuses of power by Mr. Trump stored there as well?

John Bolton might have answers. Democrats should strike a deal, even an implicit one, to get him on the witness stand.

Elizabeth Drew, a political journalist who for many years covered Washington for The New Yorker, is the author of “Washington Journal: Reporting Watergate and Richard Nixon’s Downfall.”

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