The House Foreign Affairs Committee chairman said Wednesday that John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE first told his panel in September to examine the ouster of former U.S. Ambassador to Ukraine Marie Yovanovitch Marie YovanovitchGrand jury adds additional counts against Giuliani associates Lev Parnas and and Igor Fruman Strzok: Trump behaving like an authoritarian Powell backs Biden at convention as Democrats rip Trump on security MORE, suggesting that one of President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE’s top advisers tipped Congress off to the administration’s dealings with Kyiv.

Foreign Affairs Chairman Eliot Engel Eliot Lance EngelHouse panel halts contempt proceedings against Pompeo after documents turned over Engel subpoenas US global media chief Michael Pack The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep MORE (D-N.Y.) said in a statement that he reached out to Bolton on Sept. 19, shortly after he departed from his post as Trump’s national security adviser.

“He and I spoke by telephone on September 23. On that call, Ambassador Bolton suggested to me — unprompted — that the committee look into the recall of Ambassador Marie Yovanovitch," Engel said.

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"He strongly implied that something improper had occurred around her removal as our top diplomat in Kyiv.”

Democrats have alleged that the motivation behind removing Yovanovitch — led by Trump lawyer Rudy Giuliani Rudy GiulianiDemocrats fear Russia interference could spoil bid to retake Senate Grand jury adds additional counts against Giuliani associates Lev Parnas and and Igor Fruman Juan Williams: Breaking down the debates MORE and his associates — was a way to clear any obstacles the career diplomat may have presented as they sought to begin their pressure campaign in Ukraine.

The surprise statement from Engel comes after Trump asked Wednesday why Bolton had not raised his concerns earlier about Trump’s contacts with Ukraine.

The president's attacks on his former staffer came after The New York Times reported earlier this week that Bolton’s forthcoming memoir is expected to claim that Trump used $391 million in U.S. aid to Ukraine as leverage for two politically motivated investigations, sparking a roaring debate on Capitol Hill about calling Bolton to testify as part of the ongoing Senate impeachment trial.

“Why didn’t John Bolton complain about this 'nonsense' a long time ago, when he was very publicly terminated. He said, not that it matters, NOTHING!” Trump tweeted, adding that Bolton's "nasty & untrue book" contains "classified" material.

....many more mistakes of judgement, gets fired because frankly, if I listened to him, we would be in World War Six by now, and goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security. Who would do this? — Donald J. Trump (@realDonaldTrump) January 29, 2020

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The White House is now formally trying to block the publication of Bolton's memoir.

Engel said he first chose to keep the conversation private, only sharing it with the three other committees spearheading the House impeachment investigation into allegations that Trump sought to pressure Ukraine into announcing probes that would have helped him in the 2020 election.

“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 [and Reform] Committees’ investigation into this matter, I informed my investigative colleagues,” Engel wrote. “It was one of the reasons we wished to hear from Ambassador Bolton, under oath, in a formal setting.”

His statement also comes in the midst of the Senate trial, as Democrats press Republicans to allow witnesses to be called before the upper chamber.

Democrats are particularly eager to hear from Bolton and acting White House chief of staff Mick Mulvaney Mick MulvaneyMick Mulvaney to start hedge fund Fauci says positive White House task force reports don't always match what he hears on the ground Bottom line MORE, both of whom have first hand knowledge of the Ukraine affair.

They believe these witnesses have information about the central charges Democrats have made against Trump: that he withheld vital funds — used to fight Russian-backed separatists — in order to pressure Ukrainian President Volodymyr Zelensky to commit to opening investigations into former Vice President Joe Biden Joe BidenBiden leads Trump by 36 points nationally among Latinos: poll GOP set to release controversial Biden report Can Donald Trump maintain new momentum until this November? MORE, a 2020 political rival, and Biden's son, as well as unfounded claims of Ukraine interfering in the 2016 presidential election.

Engel used the release of his statement to further push the need for Bolton to testify.

“Ambassador Bolton has made clear over the last few months that he has more to say on this issue. And now that the President has called his credibility into question, it’s important to set the record straight,” Engel added.

Before the start of the impeachment trial, Bolton publicly announced that he would be willing to testify if the GOP-controlled Senate subpoenaed him for testimony, a move that bolstered Democrats' demands for witnesses and that enhanced the spotlight on moderate Republicans who said they were undecided on the matter.

And since the leak of his unpublished book manuscript, the debate has grown even more heated.

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Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellOcasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat McConnell locks down key GOP votes in Supreme Court fight Video shows NYC subway station renamed after Ruth Bader Ginsburg MORE (R-Ky.) has been against bringing in witnesses, arguing that the Democrats should not have rushed through their impeachment inquiry when they could’ve pursued such testimony in the House.

Democrats, meanwhile, have argued that the unprecedented effort by the White House to block witnesses — both current and former administration officials across all levels — hindered their ability to bring in these firsthand witnesses.

And they also note that Bolton, a long time supporter of executive privilege, made clear that he would challenge a subpoena from the House in court, which would tie up the matter for months, time that House Democrats say they did not have.

Republicans appear increasingly confident that they will be able to block new witnesses in a vote set for Friday.

—Updated at 2 p.m.