He will also testify that he confronted Gordon D. Sondland , the United States ambassador to the European Union, the day the envoy spoke in a White House meeting with Ukrainian officials about “Ukraine delivering specific investigations in order to secure the meeting with the president.”

Even as he expressed alarm about the pressure campaign , the colonel and other officials worked to keep the United States relationship with Ukraine on track. At the direction of his superiors at the National Security Council, including John R. Bolton , then the national security adviser, Colonel Vindman drafted a memorandum in mid-August that sought to restart security aid that was being withheld from Ukraine, but Mr. Trump refused to sign it, according to documents reviewed by the Times. And he drafted a letter in May congratulating Mr. Zelensky on his inauguration, but Mr. Trump did not sign that either, according to the documents.

Colonel Vindman was concerned after he learned that the White House budget office had taken the unusual step of withholding the $391 million package of security assistance for Ukraine that had been approved by Congress. At least one previous witness has testified that Mr. Trump directed that the aid be frozen until he could secure a commitment from Mr. Zelensky to announce an investigation of the Bidens.

While Colonel Vindman’s concerns were shared by a number of other officials, some of whom have already testified, he was in a unique position. Because he emigrated from Ukraine along with his family when he was a child and is fluent in Ukrainian and Russian, Ukrainian officials sought advice from him about how to deal with Mr. Giuliani , though they typically communicated in English.

On two occasions, the colonel brought his concerns to John A. Eisenberg, the top lawyer at the National Security Council. The first came on July 10. That day , senior American officials met with senior Ukrainian officials at the White House, in a stormy meeting in which Mr. Bolton is said to have had a tense exchange with Mr. Sondland after the ambassador raised the matter of investigations he wanted Ukraine to undertake. That meeting has been described in previous testimony in the impeachment inquiry.