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The National Security Council's top Ukraine expert plans to tell House impeachment investigators on Tuesday that he was so troubled by President Donald Trump's July phone call with Ukraine's President that he reported his concerns to a superior, according to a copy of his opening statement obtained by CNN.

The expert, Army Lt. Col. Alexander Vindman, plans to say he felt an investigation into former Vice President Joe Biden and the Ukrainian natural gas company connected to Biden's son, Hunter, would undermine U.S. national security. Trump pressed Ukrainian President Volodymyr Zelensky for such an investigation multiple times during that July 25 call. There is no evidence of wrongdoing by either Joe or Hunter Biden, CNN reports.

"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 plans to tell lawmakers, according to his opening statement, which was first reported by The New York Times. "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."

That phone call between Trump and Zelensky is at the heart of the Democrats' impeachment inquiry, following a whistleblower complaint alleging that Trump had solicited foreign interference to dig up information on a political rival and the White House tried to cover it up. Vindman is the first person who was actually on the call to testify in front of House investigators.

Vindman says in his opening statement he reported his concerns about the call "internally to National Security officials in accordance with my decades of experience and training, sense of duty, and obligation to operate within the chain of command." He plans to tell investigators, "I am a patriot, and it is my sacred duty and honor to advance and defend OUR country, irrespective of party or politics."

Vindman's planned remarks also appear to contradict previous testimony from Gordon Sondland, the U.S. ambassador to the European Union, about a July 10 meeting in Washington with Ukrainian and U.S. officials.

Vindman plans to say Sondland "started to speak about Ukraine delivering specific investigations in order to secure the meeting with the President," which prompted "Ambassador (John) Bolton (to) cut the meeting short."

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"Following this meeting, there was a scheduled debriefing during which Amb. Sondland emphasized the importance that Ukraine deliver the investigations into the 2016 election, the Bidens, and Burisma," Vindman's opening 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. Dr. (Fiona) Hill then entered the room and asserted to Amb. Sondland that his statements were inappropriate."

The account stands in contrast to Sondland's testimony about the meeting wherein he stated "if Ambassador Bolton, Dr. Hill, or others harbored any misgivings about the propriety of what we were doing, they never shared those misgivings with me, then or later." Bob Luskin, an attorney for Sondland, declined to comment on the apparent contradiction.

Notably, Vindman's accounting of the meeting matches the testimony of Bill Taylor, the top U.S. diplomat in Ukraine, who told investigators Vindman and Hill had detailed the meeting to him.

Vindman began his tenure at the National Security Council in July 2018 after serving as a Foreign Area Officer specializing in Eurasia beginning in 2008. Vindman also served multiple overseas tours as an infantry officer, including a deployment to Iraq where he received a Purple Heart after being wounded in an IED attack.

House Democrats are likely interested to hear what Vindman knows about how the Trump-Zelensky call transcript was stored in the immediate aftermath of the conversation, specifically allegations by the whistleblower that it was inappropriately stored in a higher-security system rather than the typical system for presidential calls.