A White House national security official intends to tell Congress on Tuesday that he heard President Trump ask Ukrainian President Volodymyr Zelensky to investigate one of his political rivals, former Vice President Joe Biden, in a July 25 phone call.

Lt. Col. Alexander Vindman—the top Ukraine expert on the National Security Council—will be the first White House official to testify before Congress to have listened in on the call in which Trump mentioned corruption allegations against Biden and his son Hunter.

According to a draft statement of Vindman’s remarks obtained by The Daily Beast, he twice raised concerns to superiors about how Trump and other senior officials were handling Ukraine. He took his concerns to NSC lawyer John Eisenberg, once on July 10 and once again shortly after the July 25 call.

“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 wrote in his draft 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.”

Hunter Biden sat on the board of Ukrainian company Burisma Holdings while his father was in office.

Vindman will tell Congress that he never interacted directly with Trump but that he did confront Ambassador to the European Union Gordon Sondland after a July 10 meeting with Ukrainian officials.

“Amb. Sondland emphasized the importance that Ukraine deliver the investigations into the 2016 election, the Bidens, and Burisma,” Vindman will say. “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.

“Following the debriefing meeting, I reported my concerns to the NSC’s lead counsel.”

Vindman will say that he believed Trump’s demands during the July 25 call would “undermine U.S. national security. Following the call, I again reported my concerns to NSC’s lead counsel.”

He will recall how Trump refused to sign a memo he drafted in August that would re-start security aid being withheld from Ukraine. Vindman will also tell Congress how he drafted a letter congratulating Zelensky on his inauguration earlier this year, but Trump also refused to sign that.

Vindman is scheduled to appear behind closed doors before the House Intelligence, Foreign Affairs and Oversight and Reform committees in defiance of White House order not to cooperate with the impeachment inquiry. His lawyer, Michael Volkov, has not spoken publicly on the matter.