WASHINGTON >> A military officer at the National Security Council twice raised concerns over the Trump administration’s push to have Ukraine investigate Democrats and Joe Biden, according to testimony the official is prepared to deliver Tuesday in the House impeachment inquiry.

Alexander Vindman, an Army lieutenant colonel who served in Iraq and, later, as a diplomat, is prepared to tell House investigators that he listened to President Donald Trump’s call with new Ukraine President Volodymr Zelenskiy and reported his concerns to the NSC’s lead counsel.

“I was concerned by the call,” Vindman will say, according to prepared testimony obtained Monday night 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 will be the first current White House official set to appear as the impeachment inquiry reaches deeper into the Trump administration and Democrats prepare for the next, public phase of the probe.

The 20-year military officer will testify that he first reported his concerns after an earlier 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 2020 Democratic presidential candidate Joe Biden.

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.”

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, who arrived in the United States as a 3-year-old from the former Soviet Union, served in various military and diplomatic posts before joining the NSC. He was 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.

Vindman will be a key witness. He attended Zelenskiy’s inauguration with a delegation led by Energy Secretary Rick Perry, and he was part of Ukraine briefings that irritated Bolton at the White House.

He will testify that he is not the whistleblower, the still unnamed government official who filed the initial complaint over Trump’s conversation with the Ukraine president that sparked the House impeachment inquiry. He will say he does not know who the whistleblower is.

“I am a patriot, and it is my sacred duty and honor to advance and defend OUR country, irrespective of party or politics,” Vindman wrote.

“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.

The testimony is expected the day after Speaker Nancy Pelosi announced the House will vote on a resolution to affirm the impeachment investigation, set rules for public hearings and outline the potential process for writing articles of impeachment against Trump. The vote is expected Thursday.

At its core, the impeachment inquiry is looking into Trump’s call with Zelenskiy, when he asked the new Ukraine president for a “favor” in what Democrats say was a quid pro quo that could be an impeachable offense.

Pelosi’s announcement Monday came just hours after a former White House national security official defied a House subpoena for closed-door testimony, escalating the standoff between Congress and the White House over who will testify.

Charles Kupperman, who was a deputy to Bolton, failed to show up for the scheduled closed-door deposition after filing a lawsuit asking a federal court in Washington to rule on whether he was legally required to appear.

The argument advanced by Kupperman’s lawyers turns on his status as a close adviser to the president and may not be available for other administration officials who are lower down the executive branch organization chart or who did not have regular contact with Trump.

Democrats have indicated they are likely to use no-show witnesses to write an article of impeachment against Trump for obstruction of justice, rather than launching potentially lengthy court battles to obtain testimony.