An Army officer and top White House Ukraine expert is expected to tell House investigators Tuesday that he twice reported concerns about Trump's tactics in dealing with Ukraine, according to a copy of his opening statement obtained by The Hill.

Lt. Col. Alexander Vindman Alexander VindmanImpeachment witness Alexander Vindman calls Trump Putin's 'useful idiot' The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Strzok: Trump behaving like an authoritarian MORE, the highest-ranking Ukraine expert on the National Security Council, twice raised objections to his superiors about how Trump and his administration were interacting with Ukraine out of a “sense of duty," the statement says.

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“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 says in his statement. “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.”

Former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE’s son Hunter Biden sat on the board of Burisma Holdings. Trump and his team urged Ukraine to investigate Hunter Biden because of his role on the board.

Vindman will be the first White House official to testify in front of the House committees who directly witnessed the phone call between Trump and Ukrainian President Volodymyr Zelensky. He plans to testify in private Tuesday in front of the House Intelligence, Foreign Affairs, and Oversight and Reform committees.

Describing himself as a “patriot,” Vindman will say that “it is my sacred duty and honor to advance and defend our country irrespective of party or politics."

The Ukraine expert said he is not the whistleblower, but his opening statement corroborates the whistleblower’s report.

Vindman says in his opening statement that he was worried when “outside influencers” were spreading a “false narrative” about Ukraine. He also writes that he confronted Ambassador to the European Union Gordon Sondland after the ambassador spoke in a White House meeting with Ukrainian leaders about “Ukraine delivering specific investigations in order to secure the meeting with the president.”

The officer says in the statement that he wrote a memorandum in mid-August that would reinstate the military aid being withheld from Ukraine, but the president declined to sign it.

Vindman says he reported his concerns to John Eisenberg, the top lawyer at the National Security Council.

The Hill reached out to Vindman’s lawyer and the White House for comment.

Vindman’s account follows a whistleblower report detailing the call between Trump and Zelensky, where the president asked the Ukrainian leader to look into Joe and Hunter Biden. Days before this ask, the White House began withholding aid from the country.

The report sparked Speaker Nancy Pelosi Nancy PelosiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Health Care: New wave of COVID-19 cases builds in US | Florida to lift all coronavirus restrictions on restaurants, bars | Trump stirs questions with 0 drug coupon plan Overnight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds MORE (D-Calif.) to launch an impeachment inquiry in the House.

– Olivia Beavers contributed