Mr. Taylor was able to elevate the investigation above political sniping into a higher realm, one about a life-or-death struggle not just for Ukraine, but also for the United States. While Mr. Trump’s administration withheld the aid, he said, Ukrainians were dying weekly in their conflict with Russian-backed forces, with Moscow ready to pounce on the slightest sign that the United States was pulling away from Ukraine. It was not just about Ukraine’s security, he said, it was about America’s as well.

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

As for Republican claims that Mr. Biden had operated corruptly in Ukraine, the day’s other witness, the impressively bow-tied George Kent, the deputy assistant secretary of state and an expert in corruption, said there was no foundation “whatsoever” to those accusations.

Republicans offered an array of defenses, repeatedly challenging the two witnesses on the grounds that they had no direct knowledge of what the president’s orders or ideas were. They said the president was only acting prudently to protect American taxpayer interests by urging investigations into corruption in a nation with a history of self-dealing by top officials before committing American aid.

Mr. Trump’s allies said the military support was eventually approved, so no offense was committed. They ridiculed the idea that Mr. Taylor, the “star witness” for Democrats, had never even met the president, and dismissed his account and those of other witnesses as hearsay. And Republicans insisted that Democrats were seeking a do-over after the Mueller report failed to deliver a knockout blow to the president. They suggested Democrats had fired at the president and missed.