House investigators on Wednesday invited former national security adviser John Bolton to give a voluntary deposition next week as part of Democrats' impeachment inquiry, in what could be key testimony on President Trump's contacts with Ukraine.

Democrats are seeking Bolton's closed-door testimony on Nov. 7, according to a source familiar with the impeachment proceedings, a move that comes following reports that his lawyers are negotiating with three House committees about possibly testifying.

Bolton's attorney Chuck Cooper told The Hill Wednesday that his client would not appear voluntarily and would need to be subpoenaed.

While Democrats have said for weeks they want to hear from Bolton, Intelligence Committee Chairman Adam Schiff (D-Calif.) fixed a spotlight on his possible testimony on Sunday.

"Obviously he has very relevant information, and we do want him to come in and testify," Schiff said on ABC's "This Week," calling Bolton a "very important" witness.

The Intelligence chairman also said he believes the White House will seek to block Bolton from testifying.

Amid the House investigation into Trump's controversial dealings with Ukraine, Democrats 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.

Bolton was involved in some of the most explosive events related to Trump's contacts with Kiev, but it's unclear whether the Republican stalwart would defend Trump's actions or paint the president's efforts to pressure Ukrainian President Volodymyr Zelensky to open an investigation into political rivals as an abuse of power.

Bolton, who departed the White House last month amid conflicts with Trump over major foreign policy matters, is said to have raised concerns about efforts by the president and his personal attorney Rudy Giuliani to get Zelensky to investigate 2020 Democratic candidate and former Vice President Joe Biden and his son Hunter Biden.

Lt. Col. Alexander Vindman, the National Security Council's (NSC) top Ukraine expert, testified Tuesday that during a July 10 meeting, U.S. Ambassador to the European Union Gordon Sondland "started to speak about delivering the specific investigations in order to secure the meeting with the President, at which time Ambassador Bolton cut the meeting short," according to a copy of his opening remarks.

"Following this meeting, there was a scheduled debriefing during which Amb. Sondland emphasized the importance that Ukraine deliver the investigations into the 2016 election, the Bidens, and Burisma," Vindman's prepared remarks continue.

Vindman said he raised his concerns to Sondland that "his statements were inappropriate and that the request to investigate Biden and his son had nothing to do with national security, and that such investigations were not something the NSC was going to get involved in or push."

Vindman said Fiona Hill, Trump's former leading Russia expert who left voluntarily in July, also voiced her concerns to Sondland.

Hill earlier this month told House investigators during her own closed-door deposition that Bolton was so alarmed by what he heard about Trump's contacts with Ukraine that he instructed her to notify the chief NSC lawyer about Giuliani's efforts as well as Sondland and acting White House chief of staff Mick Mulvaney, The New York Times reported at the time.

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

Separately, Democrats have also requested testimony next week from two other top White House officials with insights into the Ukraine saga: John Eisenberg, senior attorney at the NSC, and one of his deputies, Michael Ellis. The pair has been asked to appear on Monday, according to a source familiar with the impeachment inquiry.

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

Upon leaving the White House, 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.

-Mike Lillis and Morgan Chalfant contributed. Last updated at 5:48 p.m.