UPDATED 3:30 PM

WASHINGTON - A White House adviser on Thursday corroborated key impeachment testimony from a senior U.S. diplomat who said last week that he was alarmed by efforts to pressure the Ukrainian government to investigate President Donald Trump's political rivals in exchange for nearly $400 million in military aid.

Tim Morrison, the top Russia and Europe adviser on Trump's National Security Council, told House investigators over eight hours of closed-door testimony that the "substance" of his conversations recalled by Bill Taylor, the acting ambassador to Ukraine, was "accurate," according to his prepared remarks and people familiar with Morrison's testimony.

In particular, Morrison verified that Trump's envoy to the European Union, Gordon Sondland, conveyed to a Ukrainian official that the military aid would be released if the country investigated an energy firm linked to the son of former vice president Joe Biden. Morrison, who announced his resignation the night before his testimony, said he did not necessarily view the president's demands as improper or illegal, but rather problematic for U.S. policy in supporting an ally in the region.

His testimony is significant given his proximity to decision-making in the White House and his status as a Trump political appointee rather than one of several career officials who in recent weeks have offered critical testimonies of Trump's Ukraine policy. Democrats hope Morrison's testimony will take away an oft-cited Republican complaint that many of the accounts from U.S. officials describing a quid pro quo are secondhand.

As Morrison gave his deposition Thursday, the House voted to formalize its impeachment inquiry, setting the stage for public hearings. The 232-196 vote split mostly along party lines.

White House press secretary Stephanie Grisham issued a statement attacking the proceedings as a violation of the president's due process and declaring that Trump did nothing wrong. "The Democrats want to render a verdict without giving the Administration a chance to mount a defense," the statement reads. "That is unfair, unconstitutional, and fundamentally un-American."

Morrison's testimony was sought because of his recurring presence in other depositions given by U.S. officials. House investigators have also requested testimony from Morrison's former boss John Bolton, the national security adviser whom Trump pushed out in September.

Morrison corroborated that he spoke with Taylor at least twice in early September. The first conversation was to alert him that Sondland had told the Ukrainians that no U.S. aid would be forthcoming until they announced an investigation of Burisma, the Ukrainian energy company that had hired Biden's son Hunter, a person familiar with Morrison's testimony said. This person, like others, spoke on the condition of anonymity to discuss the closed-door testimony.

Morrison also told lawmakers that he spoke with Taylor again on Sept. 7 to share his "sinking feeling" about a worrisome conversation between Trump and Sondland, this person said. Morrison said that, during that conversation, Trump said he was not seeking a "quid pro quo" but then went on to insist that Ukrainian President Volodymr Zelensky must publicly announce he was opening investigations of Biden and 2016 election interference. Trump has espoused a debunked theory that the campaign to undermine American democracy was carried out in Ukraine and not Russia, as the U.S. intelligence agencies unanimously concluded.

Robert Luskin, an attorney for Sondland, said Sondland never mentioned Biden by name and did not know that Burisma was linked to the vice president's son.

Despite confirming Taylor's account about the pressure Trump's associates placed on Ukraine, Morrison did not come off during his closed-door testimony as outraged or particularly troubled by the effort, said people familiar with his deposition.

Yet Morrison twice reached out to the National Security Council's attorneys with apparent concerns about Trump's conversations pertaining to Ukraine policy, according to various witness' testimony. People familiar with his deposition said Morrison reported the July 25 phone call between Trump and Zelensky - as another White House official, Lt. Col. Alexander Vindman, had done. In September, Morrison also alerted NSC lawyers about a separate conversation between Trump and Sondland, according to Taylor's testimony to Congress.

Morrison told lawmakers that he notified the lawyers because he had specific concerns that a rough transcript of Trump's call with Zelensky could be leaked. Its disclosure could prove problematic in politically polarized Washington, he said, adding that he was worried that it could affect bipartisan support for Ukraine in Congress and that it could affect how Ukraine perceived its relationship with the United States.

He said the rough transcript "accurately and completely reflects the substance of the call," which some Democratic lawmakers have challenged in recent days.

Morrison told investigators that "the White House was holding up the security sector assistance passed by Congress" from former deputy national security adviser Charles Kupperman, who directed him to design a policy to show that the U.S. government still supported giving military aid to Ukraine. Morrison said he was "confident" that the cadre of high-ranking national security officials "could convince President Trump to release the aid" because Zelensky and the Ukrainian legislature "were genuinely invested in their anti-corruption agenda."

He added, however, that he had "no reason to believe" the Ukrainians knew U.S. aid was being withheld until Aug. 28, and that he did not suspect a possible quid pro quo surrounding the aid until Sondland told him as much on Sept. 1 - information he relayed to Taylor later that day.

Morrison's testimony differs from Taylor's slightly on the details of what the arrangement entailed. Morrison recalled Sondland having told a senior Ukrainian official that the United States would be satisfied if Ukraine's prosecutor general would publicly commit to conducting the investigations Trump desired. Taylor's account said Ukraine's president would need to be the one who committed to the probes.

Morrison has been on the job for about 15 months, having joined the National Security Council during Bolton's tenure as national security adviser. In a statement on Wednesday, a senior U.S. official said Morrison "decided to pursue other opportunities - and has been considering doing so for some time."

Morrison's explanation about why he is leaving the White House has been brief, and he appears uncomfortable answering those questions, said people familiar with his testimony.

To replace Morrison, the White House has hired Andrew Peek, the State Department's deputy assistant secretary of state for Iraq and Iran, said a person familiar with the decision.

Morrison's departure from the National Security Council removes an important vestige of Bolton's tenure in the administration. Bolton handpicked Morrison to join the NSC because of his shared opposition to arms-control agreements, which both men consider an unacceptable constraint on American power. Morrison was initially brought on as the senior director for weapons of mass destruction and biodefense.

In July, Morrison replaced Fiona Hill - who also testified in the impeachment inquiry - as the president's top Russia adviser.

Taylor testified that Morrison told him Trump didn't want to provide "any assistance at all" to Ukraine.

“That was extremely troubling to me,” Taylor said, adding, “If the policy of strong support for Ukraine were to change, I would have to resign. Based on my call with Mr. Morrison, I was preparing to do so.”

--The Washington Post