But his facts led him to a pretty politically charged conclusion. “I think it’s crazy to withhold security assistance for help with a political campaign,” Mr. Taylor said. When pressed what he meant, Mr. Taylor added that because “that security assistance was so important for Ukraine as well as our own national interests, to withhold that assistance for no good reason other than help with a political campaign made no sense.”

“It was counterproductive to all of what we had been trying to do,” he added. “It was illogical. It could not be explained. It was crazy.”

The issue is explained away by Mr. Taylor’s superiors in the State Department and the White House, who argue that the story of withheld aid is a political concoction. Ultimately, the funds were released. It was like paying your credit bill on the last day possible — in this case, the deadline was the end of the government’s fiscal year on Sept. 30.

No real harm, no impeachable foul, they contend, and did not President Barack Obama decline to provide the Ukrainians with Javelin antitank missiles? One of Mr. Trump’s senior advisers noted that Washington’s press corps was not writing several years ago that Mr. Obama was, in this official's words, leaving Ukrainians to die. In contrast, Mr. Trump offered them the Javelins. (Mr. Trump’s sale of those weapons prohibited the use of Javelins on the front lines, in an effort to cast them as deterrent weapons.)

But from where Mr. Taylor was sitting in Kiev, withholding the aid hardly seemed harmless. The power of his testimony lay in how starkly he laid out what amounted to an extortion scheme: that Mr. Trump was personally refusing to release the funds unless Ukraine’s new president, Volodymyr Zelensky , publicly announced two investigations.

One was into Burisma, the energy company in which the son of former Vice President Joseph R. Biden Jr. had taken a board seat and payments of as much as $50,000 a month. The other was an investigation into a completely discredited theory that Ukrainian hackers, not Russia’s military intelligence unit, may have been responsible for the 2016 breach of the Democratic National Committee. If that was true, the Justice Department might have to consider withdrawing its indictment of a dozen Russian intelligence officials for masterminding and executing one of the boldest hackings in American political history. The indictment was issued last year by Jeff Sessions, who was then serving as attorney general.