Catherine M. Croft, who succeeded Mr. Anderson, said that she was “trepidatious” about taking a job advising Mr. Volker because of Mr. Giuliani. She feared that given his public demands for investigations by Ukraine into former Vice President Joseph R. Biden Jr. and his son, “it was possible that the Trump administration would choose to change its policy to suit domestic politics.”

Mr. Trump railed against the coming hearings on Monday, offering another dose of his combative and unrelenting response to the impeachment inquiry. On Twitter, he accused Representative Adam B. Schiff, Democrat of California and the chairman of the Intelligence Committee, of releasing “doctored transcripts” of the administration officials who testified behind closed doors, though he offered no evidence.

In fact, even the president’s most loyal Republican allies, who participated in hours of closed-door questioning of the witnesses, have refrained from raising doubts about the veracity of the transcripts that have been released so far.

But Mr. Trump said the effort to remove him from office was an “Impeachment Scam,” and he has repeatedly demanded that Republicans join him in undermining the legitimacy of the Democratic-led inquiry. In one tweet on Monday, Mr. Trump called on Republicans to “put out their own transcripts.”

In another, the president called for fraud investigations into Mr. Schiff and the C.I.A. whistle-blower who revealed a July 25 call between Mr. Trump and President Volodymyr Zelensky of Ukraine. Mr. Trump also urged an inquiry into Mark Zaid, the whistle-blower’s lawyer, who has been a critic of Mr. Trump and once tweeted that a “coup has started.”

Mr. Trump has repeatedly said that the July 25 call was “perfect.” On Monday night, Mr. Trump said in a tweet that he planned to release the transcript of an earlier call with Mr. Zelensky, claiming to be the most transparent president in history.

“I am sure you will find it tantalizing!” Mr. Trump wrote.

The transcripts of the three officials released on Monday added more detail to the narrative that Democrats are hoping to present to the public about Mr. Trump’s efforts to use the power of his office to leverage Ukraine in ways that could help him in the 2020 election.