WASHINGTON — An ambassador at the center of the House impeachment inquiry testified on Wednesday that he was following President Trump’s orders, with the full knowledge of other top administration officials, when he pressured the Ukrainians to conduct investigations into Mr. Trump’s political rivals in what he called a clear “quid pro quo.”
Gordon D. Sondland, Mr. Trump’s envoy to the European Union, told the House Intelligence Committee that he reluctantly followed Mr. Trump’s directive. He testified that the president instructed him to work with Rudolph W. Giuliani, Mr. Trump’s personal lawyer, as he pressured Ukraine to publicly commit to investigating former Vice President Joseph R. Biden Jr. and an unsubstantiated theory that Democrats conspired with Kyiv to interfere in the 2016 election.
“We followed the president’s orders,” Mr. Sondland said.
His appearance amounted to an act of defiance by an official who has been described by other witnesses as a point man in the push to extract the investigations. In his testimony, Mr. Sondland linked the most senior members of the Trump administration to the effort — including the vice president, the secretary of state, the acting chief of staff and others. He said they were informed of it at key moments, an account that severely undercut Mr. Trump’s frequent claims that he never pressured Ukraine.
Instead, Mr. Sondland, a wealthy Republican megadonor, described an expansive effort to help the president do just that.
Later on Wednesday, a Defense Department official, Laura K. Cooper, testified that Ukrainian officials may have known as early as late July that a $391 million package of security assistance was being withheld by the Trump administration.
The testimony by Ms. Cooper called into question another central element of the president’s defense: that there was no pressure because Ukrainian officials were unaware that the money was frozen.
Two months into the investigation, Mr. Sondland’s account came as close as investigators have gotten to an admission from an official who dealt directly with Mr. Trump. But Mr. Sondland’s accounts have shifted since the committee first deposed him in October, opening him up to Republican criticism that he is not credible.
Mr. Sondland has repeatedly claimed not to have recalled key episodes, and he conceded during testimony on Wednesday that he did not record precisely what had happened. He blamed the State Department for not providing him with all his emails, call logs and other records.
Still, he offered revelations and had the evidence to corroborate them.
Secretary of State Mike Pompeo signed off on parts of the pressure campaign, Mr. Sondland testified, and Mick Mulvaney, the acting White House chief of staff, was deeply involved. They understood, as he did, that there was a quid pro quo linking a White House meeting for President Volodymyr Zelensky of Ukraine to a promise by him to announce investigations into Mr. Trump’s political rivals, he said.
“I know that members of this committee have frequently framed these complicated issues in the form of a simple question: Was there a quid pro quo?” Mr. Sondland said. “As I testified previously, with regard to the requested White House call and White House meeting, the answer is yes.”
“Everyone was in the loop,” he said. “It was no secret.”
Mr. Sondland testified that he came to believe that Mr. Trump was also linking congressionally approved military assistance to Ukraine with a public commitment by Mr. Zelensky to investigate Mr. Trump’s political adversaries. Mr. Sondland said he informed Vice President Mike Pence of his concern about that connection during a Sept. 1 meeting in Warsaw.
Ms. Cooper testified that Ukrainian officials had reached out to the State and Defense Departments with questions about the status of the military funding on July 25, only hours after Mr. Trump pressed Mr. Zelensky during a phone call for the investigations. Republicans have insisted that Ukraine did not know that the aid had been held up until it was reported in the news media in late August.
Beyond the evolving timeline, Mr. Sondland’s testimony raised questions about whether the other top administration figures he mentioned — including Mr. Pompeo, Mr. Mulvaney and John R. Bolton, the president’s former national security adviser — would come forward to testify.
The Trump administration tried to block the testimony of Mr. Sondland, Ms. Cooper and David Hale, the No. 3 State Department official, who also appeared on Wednesday, and refused to allow Mr. Sondland access to certain documents, he said, which it also withheld from the committee despite a subpoena.
Democrats pointed to the administration’s stonewalling as yet another piece of evidence for an impeachment article against Mr. Trump for obstruction of Congress.
“It goes right to the heart of the issue of bribery, as well as other potential high crimes and misdemeanors,” Representative Adam B. Schiff, Democrat of California and the chairman of the Intelligence Committee, told reporters during a brief break in the hearing.