Donald Trump’s tweets attacking Marie Yovanovitch during her powerful testimony before the House impeachment inquiry has turned into a lightning rod of the hearings — and even some prominent conservative sources have slammed the president’s comments.

The tweet was highlighted by House Intelligence Committee chairman Adam Schiff, who asked Ms Yovanovitch what she thought of him saying that “everywhere Marie Yovanovitch went turned bad”, before attacking her decades of experience.

“Very intimidating,” Ms Yovanovitch said of the tweet.

The striking moment came in the latter half of Democrats’ questioning of Ms Yovanovitch, who claims she was forced out of her post as ambassador to Ukraine earlier this year after a smear campaign undertaken by Mr Trump’s personal lawyer, Rudy Giuliani.

The tweet has been derided by Democrats in Washington, and even some prominent conservatives, including Fox News host Bret Baier and Ken Starr, the former special counsel who led an impeachment-bound investigation into Bill Clinton in the 1990s.

“This whole hearing turned on a dime when the president tweeted about her real-time, and during the questioning Adam Schiff stopped the Democratic questioning to read the president’s tweet to her, and get her response,” Mr Baier said. “Now that enabled Schiff to then characterise that tweet as intimidating the witness or tampering the witness, which is a crime. Adding, essentially, an article of impeachment real-time as this hearing was going on.”

He continued: “That changed this entire dynamic of this first part of this hearing and Republicans now are going to have to take the rest of this hearing to probably try to clean that up.”

Mr Starr, who led a polarising investigation into Mr Clinton — a Democrat who was impeached after lying under oath about an affair with a White House intern — in the 1990s, also weighed in during an appearance on Fox News.