In other testimony, a senior National Security Council aide on Thursday confirmed a key episode at the center of the impeachment inquiry, testifying that a top diplomat working with Mr. Trump told him that a package of military assistance for Ukraine would not be released until the country committed to investigations the president sought.

In a closed-door deposition, the aide, Timothy Morrison, also said he had been told of a September call between Mr. Trump and the diplomat, Gordon D. Sondland. In that conversation, the president said he was not looking for a quid pro quo, but then went on to “insist” that Ukraine’s president publicly announce investigations into Mr. Biden, his son and other Democrats.

Mr. Morrison, a Trump political appointee and a former longtime Republican congressional aide, said in his opening remarks that he did not draw conclusions about Mr. Trump’s involvement in pressuring Ukraine. In subsequent testimony, he said he did not view the July phone call between Mr. Trump and Mr. Zelensky as illegal or improper, but he found it striking enough to ask the National Security Council’s chief lawyer, John Eisenberg, to review it.