In vivid and at times contentious testimony before House impeachment investigators, the senior White House official responsible for Ukraine described what he believed was an unambiguous effort by President Trump to pressure the president of Ukraine to open investigations targeting American politicians in exchange for a coveted Oval Office meeting.

Under questioning from Rep. Peter Welch (D-Vt.) and other Democrats, Lt. Col. Alexander Vindman said “there was no doubt” about what Trump wanted when he spoke by phone on July 25 to Ukrainian President Volodymyr Zelensky — particularly in contrast with an April call between the two shortly after Zelensky’s election.

“The tone was significantly different,” Vindman said, according to a transcript of his Oct. 29 deposition released Friday. Vindman went on to tell Welch, “I’m struggling for the words, but it was not a positive call. It was dour. If I think about it some more, I could probably come up with some other adjectives, but it was just — the difference between the calls was apparent.”

Welch asked Vindman if he had any doubt that Trump was asking for investigations of his political opponents “as a deliverable” — in other words, as part of a quid pro quo.

“There was no doubt,” Vindman said.

In a crucial discussion of what constitutes a quid pro quo, Vindman was grilled by a Republican member of the committee about why he believed Trump had made a “demand” that Ukraine launch an investigation of Hunter Biden in return for a White House meeting for Zelensky. Biden is the son of former vice president Joe Biden, a leading contender for the 2020 Democratic presidential nomination, and was once employed by a controversial Ukrainian energy firm.

Vindman, explaining what he called the vast “power disparity” between Trump and Zelensky, told Rep. John Ratcliffe (R-Tex.) that Trump’s request for a “favor” from Zelensky was fairly interpreted as a demand.

“When the president of the United States makes a request for a favor, it certainly seems — I would take it as a demand.”

“Fair enough,” said Ratcliffe, who went on to express doubts about the premise.

Vindman said that his reasoning was that “this was about getting a White House meeting. It was a demand for him to fulfill … this particular prerequisite to get the meeting.”

Ratcliffe pressed Vindman on the word “demand,” saying “we’re talking about an allegation that there was a quid pro quo significance, and demand has a specific connotation.” Ratcliffe stressed that Trump and others have denied there was any such demand.

But Vindman stood by his description, saying “it became completely apparent what the deliverable would be in order to get a White House meeting. That deliverable was reinforced by the President. … The demand was, in order to get the White House meeting, they had to deliver an investigation.”

Many of Vindman’s concerns about politicizing the relationship with Ukraine, which the U.S. sees as a bulwark against Russian expansion in Europe, were shared by Fiona Hill, who previously served as the top Russia policy official on the National Security Council.

Hill, whose deposition testimony also was released Friday, testified that Trump’s personal attorney, Rudolph W. Giuliani, and his business associates, Igor Fruman and Lev Parnas, were trying to use the powers of the presidency to further their own personal interests. Fruman and Parnas were arrested last month and face federal charges of funneling foreign money to U.S. politicians while trying to influence U.S.-Ukraine relations.

Even before Trump’s July 25 phone call with Zelensky, during which Trump said Ukraine’s president should be in touch with Giuliani about investigations, “there was a lot of usurpation of that power,” Hill told impeachment investigators, characterizing Giuliani and his associates as “trying to appropriate presidential power or the authority of the President, given the position that Giuliani is in, to also pursue their own personal interests.”

Hill said that, in hindsight and with the benefit of a rough transcript of the July 25 call and media reports, she believed that her “worst nightmare” for U.S.-Ukraine relations had come to pass.

“My worst nightmare is the politicization of the relationship between the U.S. and Ukraine and, also, the usurpation of authorities, you know, for other people’s personal vested interests,” Hill said. “And there seems to be a large range of people who were looking for these opportunities here.”

Mike DeBonis, Greg Jaffe, Michael Kranish, Elise Viebeck, Karoun Demirjian, Josh Dawsey, Rachael Bade, Ellen Nakashima, John Hudson, Karen DeYoung, John Wagner, Colby Itkowitz and Matt Zapotosky contributed to this report.