A U.S. Army official and White House national security official plans to tell members of Congress conducting an impeachment inquiry that he was on the phone call between President Donald Trump and Ukraine’s leader in which Trump asked for an investigation into the Bidens, and that he raised concerns about it.

Lt. Col. Alexander S. Vindman, who is the top Ukraine expert on the National Security Council, considered the request so damaging to American national security that he reported it to a superior, according to his opening statement obtained by NBC News. The New York Times first reported the contents of his written statement.

Lt. Col. Alexander Vindman, director for European Affairs at the National Security Council, arrives at the Capitol on Tuesday. Mark Wilson / Getty Images

Vindman is expected to testify Tuesday that he was on the July 25 call between President Donald Trump and Ukrainian President Volodymr Zelenskiy, making him the first witness to the call to appear before Congressional investigators in the Democrats’ impeachment inquiry.

"I was concerned by the call. 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's opening statement says.

"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. Following the call, I again reported my concerns to NSC’s lead counsel," Vindman's opening statement says.

The call is at the center of the impeachment inquiry against Trump launched by House Democrats. Critics say that Trump’s alleged pressure on the Ukrainians amounted to an abuse of power designed to help him in the 2020 presidential election.

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Vindman is the National Security Council director for European affairs. He joined the National Security Council in July of 2018.

He will tell Congress that he is not the unnamed whistle-blower whose complaint sparked what would become the impeachment inquiry now underway, according to the statement, and that he does not know who the whistle-blower is.

Vindman will also say that he twice reported his concerns to the National Security Council’s lead attorney, John Eisenberg, once after a July 10 meeting about comments made by U.S. ambassador to the European Union Gordon Sondland, and also after Trump's phone call with Zelenskiy.

On July 10, at a visit by Oleksandr Danylyuk, Ukraine’s secretary of national security, and then-national security adviser John Bolton in Washington, D.C., “Sondland started to speak about Ukraine delivering specific investigations in order to secure the meeting with the President,” and that at that time Bolton cut the meeting short.

At a debriefing that followed, “Sondland emphasized the importance that Ukraine deliver the investigations into the 2016 election, the Bidens, and Burisma,” Vindman’s statement says.

“I stated to Amb. 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,” the statement says.

Vindman’s statement says he reported his concerns to the National Security Council’s lead attorney. The statement says he also did so after the July 25 phone call.

In his prepared remarks, Vindman also says that in the spring of 2019 "I became aware of outside influencers promoting a false narrative of Ukraine inconsistent with the consensus views of the interagency," and that "this narrative was harmful to U.S. government policy."

His statement says he never had any direct communications with Trump.

Vindman served multiple overseas tours as an infantry officer and received the Purple Heart after being wounded by an improvised explosive device attack while in Iraq.

“I am a patriot, and it is my sacred duty and honor to advance and defend OUR country, irrespective of party or politics,” Vindman says in the prepared remarks, adding that he felt an obligation to report his concerns.

“On many occasions I have been told I should express my views and share my concerns with my chain of command and proper authorities. I believe that any good military officer should and would do the same, thus providing his or her best advice to leadership,” his statement says.

Vindman's scheduled testimony comes a week after Bill Taylor, the top U.S. diplomat in Ukraine, told members of Congress that Trump directed officials to tie foreign aid to Ukraine to demands that the country open an investigation into the Biden family and the 2016 election.

Trump and his defenders have repeatedly denied any quid pro quo took place. Taylor’s opening statement provided to NBC News appeared to deal that a potentially serious blow.

Trump has characterized the phone call as “perfect” and has said there was no quid pro quo. He has called the impeachment inquiry a “witch hunt” and a “fraud.”

The Democrat-controlled House of Representatives is expected to vote Thursday on a Democratic resolution that will lay out the next steps in the impeachment inquiry, according to a senior congressional source.