WASHINGTON - Three current and former Trump administration officials described Tuesday how they harbored a variety of concerns surrounding a July phone call in which President Donald Trump pressed his Ukrainian counterpart to investigate former vice president Joe Biden - boosting Democrats' inquiry into whether Trump should be impeached and substantially undercutting the president's assertion that the conversation was "perfect."

Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky has been at the heart of Democrats' impeachment investigation, and on Tuesday, they solicited public testimony from the trio of firsthand witnesses, who had been tasked with listening in.

Lt. Col. Alexander Vindman, the National Security Council's European affairs director, said he considered the president's demand of the Ukrainian leader "inappropriate," because it could have "significant national security implications" for the United States.

Jennifer Williams, Vice President Mike Pence's special adviser on Europe, said she thought the call was "unusual" because "it involved discussion of what appeared to be a domestic political matter."

And Tim Morrison, the NSC's former top Russia and Europe adviser, said he worried what might happen if the call was made public - as it ultimately was, after an intelligence community whistleblower complained about it and helped jump-start Democrats' impeachment inquiry.

"I feared at the time of the call on July 25th how its disclosure would play in Washington's political climate," Morrison said. "My fears have been realized."

The three witnesses were joined Tuesday by Kurt Volker, a former Trump administration envoy to Ukraine. Their day-long testimony kicked off what is likely to be the most intense week yet in the impeachment inquiry.

Lawmakers are scheduled to hear from nine witnesses before Friday, as they seek to build a case not just that Trump asked his Ukrainian counterpart for a political favor, but that he withheld hundreds of millions of dollars in military aid and a White House meeting to ensure he would get what he wanted.

The House Intelligence Committee will hear Wednesday from a witness who is perhaps the most critical: Gordon Sondland, ambassador to the European Union, who talked to Trump regularly and seemed to take a hands-on role in communicating the president's demands to the Ukrainians.

As they had behind closed doors, the witnesses testifying Tuesday, some in the face of public attacks from the president and his allies, offered a clear window into how Trump used the power of his office in a bid to get a political benefit from a foreign leader.

Republicans, meanwhile, intensified their attacks on the investigation - questioning Democrats' motives, scrutinizing witnesses and suggesting that Trump was merely concerned about Ukrainian corruption in general.

"The Democrats are no closer to impeachment than where they were three years ago," the House Intelligence Committee's top Republican, Rep. Devin Nunes of California, said during Tuesday's hearings.

Trump said Tuesday that impeachment was "a little pipe dream" of House Speaker Nancy Pelosi, D-Calif., and that Republicans "are absolutely killing it" with their line of questioning for the witnesses.

Vindman and Williams testified together in a morning session before the House Intelligence Committee, followed in the afternoon by Morrison and Volker. Republicans had requested Volker and Morrison as witnesses and treated them far more gently - though their remarks did not fully exonerate Trump.

Volker testified that while he was aware that the administration was holding back aid from the Ukrainians as Trump sought investigations, he was not aware of a quid pro quo. He said he believed that the president merely harbored a general view that corruption was rampant in Ukraine - a view that was not necessarily unfair, given the country's past leadership.

"The issue of the security assistance was one where I thought this was related to a general negative view about Ukraine," Volker said.

Volker said, too, that while he was involved in the administration's pressure on the Ukrainians to announce investigations of interest to the president, he did not connect those probes to Biden, Trump's political rival. He said he initially believed that the administration was pursuing investigations of potential Ukrainian interference in the 2016 election and of a Ukrainian energy company, Burisma.

Biden's son Hunter was on the board at Burisma. Volker testified that he was trying to "thread a needle" in divorcing the two and believed that pursuing an investigation of the former vice president would amount to examining "conspiracy theories that have been circulated by the Ukrainians."

"The accusation that Vice President Biden was acting inappropriately didn't seem at all credible to me," he said.

Volker was not on the July 25 call in which Trump mentioned Biden specifically, and he said he "would have objected" to pursuing such an inquiry.

All four witnesses already had provided lawmakers with private depositions, and some had since been subjected to attacks from Trump and his allies. Their public appearances suggested that while they had noted the criticism, they would not be deterred.

Williams - whom Trump tweeted about over the weekend - said she was "surprised" by the president's tweet, which suggested that she was among a group of "Never Trumpers" who were trying to launch a "presidential attack."

"I was not expecting to be called out by name," Williams said, denying that she had attempted to launch an attack on Trump.

Vindman, in his Army dress uniform, initially spoke quickly and nervously, the sheets of paper containing his opening statement shaking in his hand as he read aloud. He called the attacks on those who have appeared before lawmakers "reprehensible" and - addressing his father, who brought the Vindman family to the United States from the Soviet Union decades ago - said: "Do not worry, I will be fine for telling the truth."

Later in the hearing, though, Vindman seemed to grow more confident. At one point, he corrected Nunes after the Republican called him "Mr. Vindman," rather than by his military rank.

"It's Lieutenant Colonel Vindman, please," Vindman said. He later declared himself "never partisan" in response to accusations that he is a "never Trumper" and, when asked about Trump's attacks, asserted of his testimony: "I knew I was assuming a lot of risk."

On Tuesday, Trump said of Vindman: "I never saw the man. I understand now he wears his uniform when he goes in."

Donald Trump Jr., the president's son, tweeted: "What a joke . . . Can anyone watch this and believe that Vindman has any credibility?"

Military officials have been monitoring Vindman's personal security and are ready to move him and his family to an Army base if necessary to protect them from threats, according to a person familiar with the discussions.

Rep. Jim Jordan, R-Ohio, read a negative assessment of Vindman from Morrison, Vindman's former boss, questioning his judgment and suggesting he might leak information. The White House later tweeted some of Morrison's claim, which Vindman disputed.

During his testimony later, Morrison confirmed that he had heard concerns from other officials about Vindman's judgment, and he said he wished Vindman had complained to him first about the Trump-Zelensky call before taking his concerns to the NSC's counsel.

Officials do not seem to unanimously view the conversation as problematic. Lt. Gen. Keith Kellogg, Pence's national security adviser, said in a statement Tuesday that he had been on the call and heard "nothing wrong or improper."

Morrison said he was disappointed over the call because it was "not what we recommended the president discuss," though he said he did not believe, in real time, that Trump was making an improper request, and he evaded Democratic efforts to pin him down on the point. Morrison reported the call to a top NSC lawyer so that access to it could be restricted.

"I was hoping for a more full-throated statement of support from the president concerning President Zelensky's reform agenda," Morrison said.

Vindman testified that he immediately reported the matter to National Security Council lawyer John Eisenberg "out of a sense of duty." After that, he said, he seemed to have been excluded from some meetings to which he thought he should have been invited.

Vindman said he also told two other officials about the call: State Department official George Kent and someone in the intelligence community. The remark - and Republicans' reaction to it - prompted committee Chairman Adam Schiff, D-Calif., to pause the hearing and express concern that Vindman's naming the official could publicly identify the whistleblower who filed a complaint about the call.

Republicans objected, noting that Vindman has said he does not know the whistleblower's identity. Vindman ultimately did not reveal the name of the intelligence official in whom he confided.

"It is improper for the president of the United States to demand a foreign government investigate a U.S. citizen and a political opponent," he testified.

Asked about the forcefulness of the president's entreaty, he responded, "The culture I come from, the military culture, when a senior asks you to do something, even if it's polite and pleasant, it's not to be taken as a request." He said a rough transcript of Trump's call with Zelenksy was moved to a secure location, where fewer people would be able to access it, but he noted, "I didn't take it as anything nefarious."

While the July phone call was the focal point of much of Tuesday's testimony, the four officials also described unusual actions the United States took toward Ukraine that, even now, they said they do not fully understand.

Williams, for example, described how Trump, after speaking with Zelensky in April, had wanted Pence to attend the new Ukrainian leader's inauguration. But in May, before the inauguration date was set, she was informed by the White House chief of staff's office that Pence would not be going, per a new request from Trump, Williams testified.

She said she did not know the reason for the change. A lower-level delegation headlined by Energy Secretary Rick Perry ultimately went instead.

Williams, Vindman and Volker testified that around this time, they were aware of an effort by the president's allies, in particular his personal attorney Rudy Giuliani, to press Ukraine for a number of investigations. Unlike the others, though, Volker described how he involved himself directly in the matter - essentially trying to humor Giuliani so he could help Zelensky get a White House meeting.

"If it threads the needle between what is reasonable for Ukraine to do, and if it resets the negative perceptions held by Mr. Giuliani and then the president, then why not?" he said.

Vindman testified that at a July 10 meeting with Ukrainian officials, Sondland declared that if his foreign counterparts wanted to get a White House meeting, the Ukrainians would have to provide a "deliverable": the investigations the president wanted.

Vindman testified that the remark was so unnerving that it prompted then-national security adviser John Bolton to abruptly cut the gathering short. He said it was not "entirely clear" that Sondland was speaking for the president, but his demand seemed to have been developed after a conversation with White House Chief of Staff Mick Mulvaney.

After the meeting, Vindman said, Sondland told others that he was referring to investigations into the Bidens and Burisma. Vindman said he responded that such probes were "inappropriate - and had nothing to do with national security."

Morrison told lawmakers that his predecessor on the NSC had warned him about the "Gordon problem," referring to Sondland, and that he responded by tracking Sondland's efforts in the context of Ukraine.

Tuesday's testimony could raise questions for Sondland, who has previously claimed he did not fully understand that the president wanted an investigation of Biden.

In a video posted by CNN on Tuesday, Zelensky refused to confirm or deny whether he was prepared to publicly announce an investigation into Burisma after his July call with Trump.

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The Washington Post's Devlin Barrett, Shane Harris, Rosalind S. Helderman, John Hudson, Colby Itkowitz, Greg Jaffe, Michael Kranish, Carol D. Leonnig, Greg Miller, Ellen Nakashima, Felicia Sonmez, Elise Viebeck and John Wagner contributed to this report.

This article was written by Elise Viebeck, Karoun Demirjian, Matt Zapotosky and Colby Itkowitz, reporters for The Washington Post.