He is among the White House officials who have described the fallout as the president and his personal lawyer, Rudolph W. Giuliani, pressured Ukraine to investigate unfounded allegations about former Vice President Joseph R. Biden Jr., a Democratic rival of the president, and to help chase an unproven theory that Ukraine, not Russia, had intervened in the 2016 election. Democrats have accused the president of abusing his official powers to benefit his re-election efforts, while House Republicans argue he did nothing wrong.

The colonel first raised concerns on July 10, when he was among a group of Americans meeting at the White House with Ukrainian officials. The Ukrainians were hoping to secure a meeting with Mr. Trump for the new Ukrainian president, Volodymyr Zelensky.

Another attendee, Gordon D. Sondland, the United States ambassador to the European Union and a prominent Republican donor, “emphasized the importance that Ukraine deliver the investigation into the 2016 elections” and also move forward with a Biden-related inquiry, the colonel testified. He also said that he immediately expressed his concern to Mr. Sondland.

“I, as the representative of the N.S.C., thought it was inappropriate and that we were not going to get involved in investigations,” Colonel Vindman testified, adding, “It was kind of an uncomfortable conversation.”

Later that day, he reported his concerns to the senior lawyer at the National Security Council.

The second episode came shortly after the colonel and other aides listened in as the president held a July 25 call with Mr. Zelensky, in which Mr. Trump pressured the Ukrainian leader to carry out the investigations he sought.