After hanging up, Mr. Sondland said that Mr. Trump did not care a whit about Ukraine but was focused on “big stuff,” like getting “the Biden investigation,” according to Mr. Holmes. His account clashes directly with Mr. Sondland’s statement that he recalled “no discussions with any State Department or White House official about former Vice President Biden or his son.” He also said he did not recall participating “in any effort to encourage an investigation into the Bidens.”

Even if Mr. Sondland is now a damaged witness, he remains an important one. So far, he is the only one cooperating with the inquiry who dealt directly with Mr. Trump on Ukraine. Another witness, Kurt D. Volker, the special envoy to Ukraine, participated in an Oval Office meeting with Mr. Trump early on, but took a back seat to Mr. Sondland after that.

For Mr. Sondland, a bald, blustery 62-year-old, the inquiry has upended his work.

He used to boast to others that he could ring the president directly, complaining to one national security official that he was harder to reach than Mr. Trump. He told another colleague that Mr. Trump was often in a bad mood in the early morning. And Mr. Sondland became impatient when White House operators failed to promptly connect him to the Oval Office. Now his every exchange with the president is under scrutiny.

Mr. Sondland once described himself as a “results-oriented,” take-charge type. Now investigators are asking him why he pushed a so-called deliverable for the president — an announcement of the investigations — that other officials have said was ethically wrong and ran counter to American national security interests.

Mr. Sondland also liked to present himself as a refreshing alternative to hidebound bureaucrats. Now some of those same bureaucrats have described him as an aggressive operator who elbowed them out of the way, rejected time-honored protocols and turned his personal cellphone into a national security risk.

As national security adviser, John R. Bolton “was frustrated with Ambassador Sondland’s involvement in these issues — frankly in a lot of issues,” Timothy Morrison, a onetime top deputy to Mr. Bolton, testified. Although his concerns were widely shared, Secretary of State Mike Pompeo failed to rein in Mr. Sondland, Mr. Morrison said.

Fiona Hill, Mr. Morrison’s predecessor, told investigators that “all kinds” of foreign officials were “appearing at the gates of the White House, calling on our personal phones” and demanding meetings that Mr. Sondland had unilaterally promised to them.