John Bolton urged congressional Democrats to investigate President Trump’s ouster of the U.S. ambassador to Ukraine, according to a senior lawmaker.

“Ambassador Bolton suggested to me, unprompted, that the committee look into the recall of Ambassador Marie Yovanovitch,” House Foreign Affairs Chairman Eliot Engel, a New York Democrat, said on Wednesday. “He strongly implied that something improper had occurred around her removal as our top diplomat in Kyiv.”

That conversation took place on Sept. 23, according to Engel. The revelation depicts Trump’s former national security adviser encouraging an investigation of the president one day before Engel and other Democratic leaders began the impeachment probe.

“At the time, I said nothing publicly about what was a private conversation, but because this detail was relevant to the Foreign Affairs, Intelligence, and Oversight Committee's investigation into this matter, I informed my investigative colleagues," Engel said.

Yovanovitch became the first diplomatic casualty of the Ukraine controversy when Trump decided to recall her from Kyiv after Rudy Giuliani and a Ukrainian prosecutor accused her of being a political enemy of the White House.

One of Trump's Republican allies panned Engel's revelation as adding little to the impeachment conversation.

"This is hardly groundbreaking or relevant," Rep. Mark Meadows, a North Carolina Republican, tweeted. "The more noteworthy item is that this is magically coming out 127 days into this impeachment process — on the eve of President Trump being acquitted, as Democrats rapidly lose momentum. You can smell their desperation."

Trump denounced Yovanovitch in a phone call with Ukrainian President Volodymyr Zelensky when he urged the new Ukrainian leader to work with Giuliani, who sought to implicate Joe Biden in a corruption scandal.

“Why didn’t John Bolton complain about this ‘nonsense’ a long time ago, when he was very publicly terminated,” Trump tweeted early Wednesday morning. “He said, not that it matters, NOTHING!”

Engel cited Trump's tweet to justify revealing the private conversation with Bolton amid a political fight over whether Bolton should testify before the Senate during the impeachment trial. Leaked excerpts of Bolton’s forthcoming memoir of his time in the White House have raised Democratic hopes that his testimony would damage Trump and make it harder for Senate Republicans to vote to acquit the president.

“Ambassador Bolton has made clear over the last few months that he has more to say on this issue,” Engel said. “It’s telling that, of all people, John Bolton is now the target of right-wing ire. It underscores just how important it is that the Senate subpoena Ambassador Bolton as a witness.”