As Marie Yovanovitch answered questions on Capitol Hill, portraying a devastating image of a state department that had failed to support her in her work, and saying she had been threatened by the president’s comments about her to a foreign leader, Mr Trump launched a series of attacks against her on social media.
“Everywhere Marie Yovanovitch went turned bad. She started off in Somalia, how did that go,” said Mr Trump.
“Then fast forward to Ukraine, where the new Ukrainian President spoke unfavourably about her in my second phone call with him. It is a US president’s absolute right to appoint ambassadors.”
He added: “They call it “serving at the pleasure of the president”.
Congressman Adam Schiff, Democratic chair of the House intelligence committee, asked Mr Yovanovitch what she thought of the president’s comments on Twitter.
“Ambassador, you’ve shown the courage to come forward today and testify. Notwithstanding the fact that you were urged by the White House or state department not to, notwithstanding the fact that as you testified earlier the president implicitly threatened you in that call record, and now the president — in real time — is attacking you,” said Mr Schiff.
“What effect do you think that has on other witnesses willingness to come forward and expose wrongdoing.”
Ms Yovanovitch, who was recalled from Ukraine where she was serving as ambassador in May at the behest of Mr Trump and Mr Giuliani, answered: “It’s very intimidating.”
Mr Schiff continued: “It’s designed to intimidate, is it not?”
She replied: “I mean, I can’t speak to what the president is trying to do, but I think the effect is trying to be intimidating.”
Mr Schiff then replied: “Some of us here take witness intimidation very seriously.”
During a break in proceedings, Mr Schiff said he thought Mr Trump’s tweets were a clear attempt to intimidate witnesses. He said: “What we saw today: witness intimidation in real time by the president of the United States.”
Mr Trump’s attack on Ms Yovanovitch followed an attack on two earlier witnesses, George Kent and William Taylor, whom he denounced during a rally in Louisiana on Thursday night.