Lt. Col. Alexander Vindman, the National Security Council staffer set to deliver dramatic testimony confirming that US President Donald Trump sought dirt on a political rival from Ukraine, is a Jewish refugee from that country when it was part of the Soviet Union.

“I did not think it was proper to demand that a foreign government investigate a US citizen, and I was worried about the implications for the US government’s support of Ukraine,” Vindman, an army officer and the top NSC official handling Ukraine, says in testimony posted Monday evening by the New York Times, which he is set to deliver to congressional investigators on Tuesday.

Vindman, who listened in on the conversation in his official capacity, would be the first whistleblower to have firsthand knowledge of the call.

“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,” he said in his prepared testimony. Burisma is a mining concern that for a period employed Biden’s son, Hunter. Allegations by Trump and others that Biden and his son were engaged in corrupt behavior have not been substantiated.

Vindman earned a purple heart when he was wounded in Iraq.