WASHINGTON — Defying White House orders, an Army officer serving with President Donald Trump’s National Security Council testified to impeachment investigators Tuesday that he twice raised concerns over the administration’s push to have Ukraine investigate Democrats and Joe Biden.

Alexander Vindman, a lieutenant colonel who served in Iraq and later as a diplomat, is the first official to testify who actually heard Trump’s July 25 call with new Ukrainian President Volodymyr Zelenskiy. He reported his concerns to the NSC’s lead counsel, he said in his prepared remarks.

His arrival in military blue, with medals, created a striking image at the Capitol as the impeachment inquiry reached deeper into the White House.

“I was concerned by the call,” Vindman said, according to his testimony obtained by The Associated Press. “I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government’s support of Ukraine.”

Vindman, a 20-year military officer, added to the mounting evidence from other witnesses — diplomats, defense and former administration officials — who are corroborating the initial whistleblower’s complaint against Trump and providing new details ahead of a House vote in the impeachment inquiry.

“Every person has put it in higher resolution,” said Rep. Denny Heck, D-Wash., during a break in the daylong session.

“That’s the story: There’s not like a new headline out of all of these,” said Rep. Tom Malinowski, D-N.J. “Every single witness, from their own advantage point, has corroborated the central facts of the story we’ve heard.”

The inquiry is looking into Trump’s call, in which he asked Zelenskiy for a “favor” — to investigate Democrats — that the Democrats say was a quid pro quo for military aid and could be an impeachable offense.

With the administration directing staff not to appear, Vindman was the first current White House official to testify before the impeachment panels. He was issued a subpoena to appear.

Trump took to Twitter Tuesday to denounce the probe as a “sham,” adding: “Why are people that I never even heard of testifying about the call. Just READ THE CALL TRANSCRIPT AND THE IMPEACHMENT HOAX IS OVER!”

Vindman, who arrived in the United States as a 3-year-old from the former Soviet Union, said that it was his “sacred duty” to defend the United States.

Some Trump allies, looking for ways to discredit Vindman, questioned the colonel’s loyalties because he was born in the region. But the line of attack was rejected by some Republicans, including Rep. Liz Cheney, who said it was “shameful” to criticize his patriotism.

Sen. Mitt Romney, R-Utah called the slams on Vindman “absurd, disgusting and way off the mark. This is a decorated American soldier and he should be given the respect that his service to our country demands.”

The testimony came the day after Speaker Nancy Pelosi announced the House would vote on a resolution to set rules for public hearings and a possible vote on articles of impeachment.

Thursday’s vote would be the first on the impeachment inquiry and aims to nullify complaints from Trump and his allies that the process is illegitimate and unfair.

White House Press Secretary Stephanie Grisham said the resolution merely “confirms that House Democrats’ impeachment has been an illegitimate sham from the start as it lacked any proper authorization by a House vote,”

Senate Majority Leader Mitch McConnell said he and other GOP lawmakers will review the resolution to see if it passes a “smell test” of fairness to Trump.

The session Tuesday grew contentious at times as House Republicans continued trying to unmask the still-anonymous whistleblower and call him or her to testify. Vindman said he is not the whistleblower and does not know who it is.

GOP Rep. Jim Jordan of Ohio acknowledged Republicans were trying to get Vindman to provide the names of others he spoke to after the July 25 phone call, in an effort to decide whom to call to testify. “He wouldn’t,” Jordan said.

In his prepared remarks, Vindman testified that in spring of this year he became aware of “outside influencers” promoting a “false narrative of Ukraine” that undermined U.S. efforts, a reference in particular to Trump’s personal lawyer, Rudy Giuliani.

He first reported his concerns after a July 10 meeting in which U.S. Ambassador to the European Union Gordon Sondland stressed the importance of having Ukraine investigate the 2016 election as well as Burisma, a company linked to the family of Biden, a 2020 Democratic presidential candidate.

Vindman says he told Sondland that “his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security, and that such investigations were not something the NSC was going to get involved in or push.”

That differs from the account of Sondland, a wealthy businessman who donated $1 million to Trump inauguration and testified before the impeachment investigators that no one from the NSC “ever expressed any concerns.” Sondland also testified that he did not realize any connection between Biden and Burisma.

For the call between Trump and Zelenskiy, Vindman said he listened in the Situation Room with colleagues from the NSC and Vice President Mike Pence’s office. He said he again reported his concerns to the NSC’s lead counsel.

He wrote, “I realized that if Ukraine pursued an investigation into the Bidens and Burisma, it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained. This would all undermine U.S. national security.”

Vindman served as the director for European affairs and a Ukraine expert under Fiona Hill, a former official who testified earlier in the impeachment probe. Hill worked for former national security adviser John Bolton.

He told investigators that Ukraine, in trying to become a vibrant democracy integrated with the West, is a bulwark against overt Russian aggression.

Vindman attended Zelenskiy’s inauguration with a delegation led by Energy Secretary Rick Perry, and he and Hill were both part of a Ukraine briefing with Sondland that others have testified irritated Bolton at the White House.

“I am a patriot, and it is my sacred duty and honor to advance and defend OUR country, irrespective of party or politics,” wrote Vindman, who was wounded in Iraq and awarded a Purple Heart.

“For over twenty years as an active duty United States military officer and diplomat, I have served this country in a nonpartisan manner, and have done so with the utmost respect and professionalism for both Republican and Democratic administrations,” he wrote.

Associated Press writers Zeke Miller, Padmananda Rama, Matthew Daly, Laurie Kellman, Eric Tucker and Alan Fram contributed to this report.