Democrats are eyeing John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE as a potential witness in their impeachment case amid increasing scrutiny of President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE and Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE’s contacts with Ukraine.

Bolton, Trump’s third national security adviser who was pushed out last month, has long clashed with Democrats over a host of foreign policy initiatives.

But amid the Democrats’ investigation into Trump’s controversial dealings with Ukraine, they now see the military hawk as a potential star witness — one whose intimate knowledge of the Ukraine affair could expose more evidence of wrongdoing by the president.

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“When he calls Giuliani a ‘live hand grenade,’ that says something. He speaks from experience. He's someone who should know,” Rep. Stephen Lynch Stephen Francis LynchOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Official: Pentagon has started 'prudent planning' for full Afghanistan withdrawal by May Overnight Defense: Dems divided on length of stopgap spending measure | Afghan envoy agrees to testify before House panel | Trump leans into foreign policy in campaign's final stretch MORE (D-Mass.), a member of the House Oversight and Reform Committee, told CNN on Tuesday, referencing Fiona Hill’s reported closed-door testimony on Monday.

“So, yes, we would want to talk to Bolton. We understand that he did leave the White House under stressful circumstances, but I think he had a good read on what was going on in Ukraine and his testimony would be very desirable as far as the committee goes,” Lynch added.

Others also echoed this position, arguing that Bolton’s conservative bona fides make him a potentially more politically potent witness — a Republican who could challenge the narrative of Trump’s GOP defenders on Capitol Hill.

“He’s as big a hawk as you’re going to find,” said Rep. Peter Welch Peter Francis WelchShakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' Vermont Rep. Peter Welch easily wins primary Vermont has a chance to show how bipartisanship can tackle systemic racism MORE (D-Vt.), a member of the House Intelligence Committee. “And according to the news report, he was shocked and appalled at the run around of the official channels for a private foreign policy run by Giuliani. So that makes what he has to say of interest to me.”

“I think it would be useful to hear from John Bolton,” echoed Rep. Gerry Connolly Gerald (Gerry) Edward ConnollyJudge issues nationwide injunction against Postal Service changes House panel advances bill to ban Postal Service leaders from holding political positions Shakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' MORE (D-Va.), a member of both the House Oversight and Reform and Foreign Affairs committees.

Bolton, who departed the White House last month amid conflicts with Trump over major foreign policy matters, is said to have raised concerns about the president and Giuliani’s efforts to get Ukrainian President Volodymyr Zelensky to investigate one of Trump’s top political opponents, 2020 Democratic candidate Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE.

Although Giuliani, Trump’s personal lawyer, is not a government employee, he has said repeatedly that he was taking direction from the State Department in efforts to drum up foreign-led anti-corruption investigations into Trump’s political rivals.

Hill, Trump’s former leading Russia expert who left voluntarily in July, told House investigators during a closed-door deposition Monday that Bolton was so alarmed by what he heard about Trump’s contacts with Ukraine that he instructed her to notify the chief lawyer for the National Security Council about the efforts of Giuliani, Ambassador to the European Union Gordon Sondland and acting White House chief of staff Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE, The New York Times reported late Monday morning.

“I am not part of whatever drug deal Sondland and Mulvaney are cooking up,” Bolton told Hill, according to her reported testimony.

If Bolton agrees to testify, he would follow a string of witnesses who have defied the White House’s order seeking to block former and current administration officials from testifying about their time in the administration, including those who have had an involvement in the Ukraine scandal.

Last week, Marie Yovanovitch, former U.S. ambassador to Ukraine, defied the administration to give nearly 10 hours of testimony, voicing concerns that her removal was politically motivated.

Hill’s appearance Monday also bucked the administration’s wishes. And George Kent, who remains at the State Department as a deputy assistant secretary, testified Tuesday even after the agency tried to prevent his appearance.

While multiple members on the Intelligence Committee said they were not sure whether the panel has been in touch with Bolton about securing a deposition, Democrats say they are racing the clock to collect as much evidence as they can about Trump’s conduct with Ukraine.

Members on the House Intelligence, Oversight and Foreign Affairs committees for the most part remained mum as they left the closed-door hearing with Kent on whether they wanted to hear from Bolton.

Some also dodged questions about whether they had been instructed to stay quiet about future witnesses, deferring to House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.), who is leading the investigation into the Trump administration’s contacts with Ukraine.

Bolton, now a Republican operative, clashed with Trump on major policy issues such as North Korea, Iran and Afghanistan, with the president viewing the longtime hawk as too militant in his approaches. He is reportedly working on a book about his time in the administration.

Still, at the onset of his departure, Bolton showed he was willing to fight back with the administration, disputing Trump’s claims that he had been fired, rather than offering to submit a letter of resignation a day prior.