So many of the defenses he floated early on have crumbled under the weight of subsequent revelations.

He started out insisting that his July 25 phone call with President Volodymyr Zelensky of Ukraine, a central element of the impeachment inquiry, was “perfect” — only to have the notes on the conversation released by the White House reveal that he had told Mr. Zelensky to open a (baseless) investigation of a political rival, former Vice President Joe Biden, whose son had done business in Ukraine.

Mr. Trump has tried to spin what he did as a good thing. “As the President of the United States, I have an absolute right, perhaps even a duty, to investigate, or have investigated, CORRUPTION, and that would include asking, or suggesting, other Countries to help us out!” he tweeted on Oct. 3.

Except that, as members of his own administration continue to clarify, this wasn’t a broad effort to root out corruption. It was a targeted campaign to pressure a foreign government to interfere with an American election on Mr. Trump’s behalf — apparently by holding hostage nearly $400 million in military aid.

He and many of his defenders have clung to the idea that there was no “quid pro quo,” a position rebutted on Tuesday by the testimony of William Taylor, the top American diplomat i n Ukraine.

Faced with this jaw-dropping account from the president’s own envoy, the White House press secretary, Stephanie Grisham, dismissed it as part of “a coordinated smear campaign from far-left lawmakers and radical unelected bureaucrats waging war on the Constitution.”

On Wednesday the president went further. He called Mr. Taylor “Never Trumper Diplomat Bill Taylor” in one tweet and declared in another: “The Never Trumper Republicans, though on respirators with not many left, are in certain ways worse and more dangerous for our Country than the Do Nothing Democrats. Watch out for them, they are human scum!”