Just days after Mr. Giuliani’s breakfast with Mr. Volker and the follow-up phone call with Mr. Yermak, Mr. Trump spoke on the telephone with Mr. Zelensky. After the Ukrainian president described his need for more American assistance against Russia, Mr. Trump asked him to “do us a favor, though” and look into Democrats.

Mr. Volker was not on that call, and he was neither shown a copy of the transcript reconstructed from the conversation nor told that the president mentioned Mr. Biden, according to one person informed about the series of events. Mr. Volker participated in Mr. Trump’s meeting with Mr. Zelensky on the sidelines of the United Nations General Assembly session this week in his last official duty.

“Kurt was one of the good ones who went in to the administration to stave off disaster,” said Thomas Wright, a senior fellow at the Brookings Institution. “They all have to speak out now about everything they know and let the chips fall where they may.”

Mr. Volker, a former career foreign service officer who represented President George W. Bush at NATO and now serves as the executive director of the McCain Institute for International Leadership at Arizona State University based in Washington, spent much of the year trying to bring Mr. Trump together with Mr. Zelensky to bolster the government elected in April.

He argued to Trump administration officials that Mr. Zelensky was a credible reformer and serious figure who could be his country’s last chance to get its act together in the face of Russian aggression. It was an uphill task, given Mr. Trump’s open disdain for Ukrainians; “they’re all corrupt and they tried to take me down,” he said in a private meeting last spring.

After the Ukrainian inauguration, Mr. Trump agreed to meet with Mr. Zelensky, but his staff kept delaying putting a date on the calendar. Like other officials, Mr. Volker was surprised to learn that Mr. Trump had ordered $391 million in aid to Ukraine frozen.

But he kept working to bring the two presidents together. Finally, the White House agreed to schedule a meeting between Mr. Trump and Mr. Zelensky during the American president’s visit to Warsaw, only to scratch the meeting when Mr. Trump decided to stay home to monitor a hurricane.