"Everywhere Marie Yovanovitch went turned bad," Trump posted on Twitter, listing her diplomatic service for the US government in Somalia and then Ukraine. Her career spanned more than three decades, going back to her first posting in Mogadishu.
Adam Schiff, the Democratic chairman of the House Intelligence Committee, blasted the president. "This is witness intimidation in real time. And we won't stand for it," he said.
The tweet was dispatched during a sensitive section of the hearing, as Yovanovitch was relating that she felt threatened when Trump disparaged her as "bad news" and told Ukrainian President Volodymyr Zelensky: "She's going to go through some things."
Asked about her reaction, she said: "Shocked. Appalled. Devastated that the president of the United States would talk about any ambassador like that to a foreign head of state. And it was me. I mean, I couldn't believe it."
She said someone told her "the colour drained from my face" as she read the transcript of the call.
Democrats are hoping the ousted ambassador's testimony injects a personal element into the impeachment inquiry to bolster their case that the president trampled over the foreign service to better his own political position and 2020 campaign.
"There has never been a disgrace like what is going on right now," Trump told reporters as the hearing took place, denying that his words could be construed as intimidation. He insisted on the right to free speech.
Yovanovich maintained a largely steady voice as she spoke about a "smear campaign" against her that appeared to have been orchestrated by allegedly corrupt individuals in Ukraine and allies of Trump, including his personal lawyer, Rudy Giuliani.
Previous testimony in the inquiry, as well as Giuliani's own public statements, have painted a picture of the lawyer running a back-door diplomatic channel, possibly to dig up dirt on Biden, and also to oust Yovanovich who refused to go along with this effort.
She also described how she was recalled from Kiev in the middle of the night after being told by US officials they were "no longer able to protect me," apparently under directions from the White House. "It was terrible. It was not the way I wanted my career to end."
Notably, the former ambassador warned that US foreign policy in Ukraine was in "disarray" and spoke more broadly about a "crisis" with the State Department, which she said was being hollowed out from within by the current leadership.
She warned of a "chilling effect" hurting diplomats. Yovanovich said the president had the prerogative to fire her, but "I do wonder why it was necessary to smear my reputation."
Just as the hearing was getting under way, the White House released a reconstructed transcript of an April call between Trump and Zelensky, who was president-elect of Ukraine at the time. The benign call appears to have been released as a tool for Trump's allies to declare him vindicated.
In the transcript of the call released on Friday, Trump invites Zelensky to the White House.
The US president's opponents say this visit was later conditioned upon Zelensky opening an investigation into Biden and his son, Hunter Biden. Democrats are also concerned the president made military assistance to Ukraine dependent on an investigation.
Trump denies any wrongdoing. The White House had previously released a July call with Zelensky - by then fully president - which Trump describes as a "perfect call."
His opponents say the transcript provided evidence - on top of diplomats' testimonies - that the president was pressuring Ukraine to investigate Biden, asking his counterpart in Kiev for a "favour," before agreeing to discuss military equipment sales.
That July call caused alarm within government, leading to a whistleblower complaint that helped spark the impeachment inquiry.
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