Washington • The White House attacked its own top Ukraine expert Tuesday as he offered sworn testimony before the House’s impeachment inquiry that President Donald Trump’s request to Ukraine’s president to investigate Democratic rivals had been “inappropriate” and validated his “worst fear” that American policy toward that country would veer off course.

On the opening day of a packed week of impeachment testimony, the expert, Lt. Col. Alexander S. Vindman, who serves on the National Security Council, said he was so alarmed by the request as he listened in to a call on July 25 between Trump and President Volodymyr Zelenskiy of Ukraine that he reported his concerns to White House lawyers. On the call, Trump pressed Zelenskiy to investigate former Vice President Joe Biden and an unproven theory that Democrats conspired with Russia to interfere in the 2016 election.

“I couldn’t believe what I was hearing,” Vindman, an Iraq War combat veteran who appeared before the House Intelligence Committee dressed in his deep-blue Army dress uniform covered with military ribbons. “It was probably an element of shock — that maybe, in certain regards, my worst fear of how our Ukraine policy could play out was playing out, and how this was likely to have significant implications for U.S. national security.”

Sitting beside him, a second White House official who also listened in on the call, Jennifer Williams, testified that she found Trump’s phone call with Zelenskiy “unusual and inappropriate,” saying she was struck that Trump was pressing a foreign leader about a personal domestic political concern.

Both witnesses testified that it was clear to the Ukrainians that the United States was withholding vital military assistance, saying that Trump administration officials had questioned the legality of doing so. And both said that no national security official in the administration supported the freeze in aid.

(Andrew Harnik | AP) Jennifer Williams, an aide to Vice President Mike Pence, and National Security Council aide Lt. Col. Alexander Vindman stand as they take a break in hearing before the House Intelligence Committee on Capitol Hill in Washington, Tuesday, Nov. 19, 2019.

Williams, a national security aide to Vice President Mike Pence, recounted a September meeting between Pence and Zelenskiy in which the Ukrainian president explained in dramatic terms how failing to provide the money would only help Russia.

“Any signal or sign that U.S. support was wavering would be construed by Russia as potentially an opportunity for them to strengthen their own hand in Ukraine,” Williams said, relaying what Zelenskiy told Pence.

After two days of earlier hearings laid out the contours of a broad pressure campaign on Ukraine by Trump and his allies, the accounts by Vindman and Williams brought the public phase of the impeachment inquiry inside the White House for the first time. For Vindman in particular, the testimony amounted to a remarkable act of public criticism of the president by a White House employee.

The colonel, who came to the United States as a refugee at 3, referred to his family’s history in Ukraine, a former Soviet republic, noting that in Russia, “offering public testimony involving the president would surely cost me my life.”

Addressing his father, who he credited with “the right decision” in leaving the Soviet Union to seek refuge in the United States 40 years ago, Vindman said: “Do not worry, I will be fine for telling the truth.”

But the White House and its Republican allies on Capitol Hill moved quickly to try to discredit Vindman, questioning his loyalty to the country and his professionalism.

As he sat in the stately House Ways and Means Committee room that is the backdrop for the impeachment hearings, the official, taxpayer-funded Twitter account of the White House posted a critical quote in which Timothy Morrison, his former boss at the National Security Council, questioned Vindman’s “judgment.”

Morrison, the council’s former senior director for Russia and Europe, was testifying in a second session Tuesday afternoon alongside Kurt D. Volker, the former U.S. special envoy to Ukraine.

Rep. Jim Jordan of Ohio, a top Republican ally of the president, cited Morrison’s comment and criticism from Fiona Hill, Vindman’s former boss at the National Security Council.

“Any idea why they have those impressions?” Jordan inquired. Vindman, who apparently came prepared for the criticism, pulled out a copy of the performance evaluation Hill wrote about him in July and read aloud from it and pressed ahead with his account of what transpired.

With his Ukrainian heritage and military background, Vindman presented a striking figure to investigators, who have already heard his account and that of Williams behind closed doors. In his testimony Tuesday, he spoke of a sense of duty and patriotism to offer his account — an implicit rebuke to conservative commentators who questioned his loyalty to the United States in recent weeks as he emerged as a public figure.

Soft-spoken at first, Vindman grew more confident in addressing lawmakers who criticized him as the hearing went on.

“Ranking member, it’s Lieutenant Colonel Vindman, please,” he instructed the committee’s top Republican, Rep. Devin Nunes of California, at one point after he addressed him as “Mr. Vindman.”

In another exchange that touched on Vindman’s loyalty to the United States, Steve Castor, the top Republican staff lawyer, asked him about three instances when Oleksandr Danylyuk, the director of Ukraine’s national security council, had approached him with offers to become the defense minister in Kyiv.

Vindman confirmed the offers and testified that he repeatedly declined, dismissing the idea out of hand and reporting the approaches to his superiors and to counterintelligence officials.

“Every single time, I dismissed it,” he said, adding that “I’m an American. I came here when I was a toddler. And I immediately dismissed these offers, did not entertain them.”

Republicans, searching for clues about the anonymous whistleblower whose account of the Ukraine matter helped launch the impeachment inquiry, pressed Vindman to recount who he spoke with about the call in its immediate aftermath. Democrats and the witness’ lawyer objected to questioning that may identify the whistleblower, but Vindman indicated that he had spoken to an intelligence official he communicated with in the normal course of business.

“Per the advice of my counsel, I’ve been advised not to answer the specific questions about members of the intelligence community,” Vindman said when pressed further.

Democrats fumed, accusing Republicans of sliming a patriot because he had a politically inconvenient story to tell.

”They’ve accused you of espionage and dual loyalties,” said Rep. Jim Himes, D-Conn. “The three minutes we asked about the offer making you minister of defense — that may have been cloaked in a Brooks Brothers suit, but that was designed exclusively to give the right-wing media an opening to questioning your loyalties.”

Fox News quickly seized on the line of questioning, with a blaring headline on its website: “Vindman says Ukrainian official offered him the job of Ukrainian defense minister.”

Democrats sought to redirect attention back to actions by Trump.

Vindman and Williams both testified that withholding the military assistance from Ukraine was damaging to relations between the two countries and to Ukraine’s ability to confront Russian aggression.

It may be too early to fully know the impact the hearings are having on public opinion. Television ratings and opinion polls released in recent days suggest that public engagement has so far fallen short not only of hearings at the height of Watergate and the impeachment of Bill Clinton, but of earlier blockbuster Trump-era congressional hearings. And after 2 1/2 days of damning public testimony, House Republicans appear to be holding together in Trump’s corner, either unconvinced his behavior was as the witnesses described or unconvinced that it warrants a remedy as drastic as impeachment.

Volker, who served until this fall as the U.S. special envoy to Ukraine, was testifying that he was left in the dark about key elements of a pressure campaign on Ukraine as described by other witnesses. Volker was the first witness to speak in private to investigators in early October, and his statement then that he was aware of no quid pro quo between the two nations has since come under scrutiny.

Volker plans to try to reconcile his earlier remarks with facts relayed by other witnesses, including Sondland and Morrison. Specifically, Volker planned to testify that he was not aware Sondland had told Ukrainian officials that the release of security assistance money was linked to a public commitment to investigate the issues Trump wanted scrutinized.

And he will state that he does not contest the accounts of other witnesses, like Vindman, of the July 10 White House meeting. Volker was present that day with Sondland, Hill and John Bolton, then the national security adviser, but he will say he may have missed the exchanges in question when Sondland tied a White House meeting for Zelenskiy to the investigations.

Depending on how forcefully Volker insists on those views, his public testimony could complicate the defense offered by Republicans, who have held his earlier account up for weeks as evidence that Trump’s conduct was not nearly so offensive as Democrats argued it was.

Morrison’s testimony may be more politically contested. In closed questioning, he confirmed key facts about Sondland’s statements about preconditions for releasing the Ukrainian assistance funds. And he recounted that it was the “unanimous” view of the White House, Defense Department and State Department national security apparatus that the assistance funds never should have been suspended and should have been reinstated immediately.

But as Republicans frequently pointed out during the morning session, he also questioned Vindman’s judgment, and testified that he saw nothing untoward to the July 25 call between Trump and Zelenskiy at the center of the inquiry.