Ms. Yovanovitch's public testimony, which played out over more than five hours in a packed and hushed House Ways and Means Committee Room, was an indictment of foreign policy in the Trump era, outlining the harm to American diplomacy and national security by a president who embraced false claims to target his own officials representing the United States overseas.

"Everywhere Marie Yovanovitch went turned bad," Mr. Trump wrote on Twitter, at the very moment that Ms. Yovanovitch was testifying about having felt threatened by the president. "She started off in Somalia, how did that go? Then fast forward to Ukraine, where the new Ukrainian President spoke unfavorably about her in my second phone call with him. It is a U.S. President's absolute right to appoint ambassadors."

Representative Adam B. Schiff, Democrat of California and the chairman of the Intelligence Committee, interrupted his counsel's questioning to read the president’s words aloud to Ms. Yovanovitch and to ask for her reaction. There were audible gasps in the room as he did so.

"It's very intimidating," she replied, taken aback.

To that, Mr. Schiff replied gravely, "Some of us here take witness intimidation very, very seriously."