Donald Trump’s former national security adviser, John Bolton, is likely to be called as a witness in the intensifying House impeachment inquiry, and no one -- including the president -- can be sure what he’ll say.

Multiple witnesses have testified that Bolton -- who was ousted from the White House last month -- expressed open contempt for efforts to seek politically motivated investigations from Ukraine that are now the heart of the House investigation.

The testimony, punctuated by a damaging account from the current U.S. envoy to Ukraine, William Taylor, on Tuesday, underscores that a pivotal witness remains waiting in the wings. Some House Democrats have already called for Bolton to appear in the inquiry, likely expecting that his acrimonious departure from Trump’s administration means he’ll feel free to speak out against his former boss.

But so far Democrats haven’t said they’ll subpoena Trump’s former top security adviser, reflecting concern that he maintains some fealty to the White House.

It’s becoming increasingly difficult for Democrats to claim they can fully investigate the president’s Ukraine campaign without speaking to Bolton. In testimony Tuesday, Taylor repeatedly referenced Bolton’s concern over what Taylor described as an “irregular policy channel” on Ukraine established by Trump associates led by Rudy Giuliani.

July 10 Meeting

“His departure in the midst of all this, I think, is significant,” Representative David Cicilline, a Rhode Island Democrat, said of Bolton on Tuesday. He added that “I would certainly welcome the opportunity” to hear the former Trump adviser’s testimony.

In one pivotal incident described by both Taylor and former National Security Council aide Fiona Hill, an irritated Bolton abruptly ended a July 10 White House meeting with a representative of Ukraine’s government after Trump’s ambassador to the European Union, Gordon Sondland, “connected ‘investigations’ with an Oval Office meeting” sought by Ukraine’s new president, Volodymyr Zelenskiy.

Hill said that Bolton described the effort to force Ukraine to investigate Trump’s political opponents as a “drug deal.” Bolton directed Hill to brief NSC lawyers about the scheme, she and Taylor testified. And Bolton opposed arranging the July 25 call between Trump and Zelenskiy, predicting it “would be a disaster,” Taylor testified.

In the call, Trump asked Zelenskiy to investigate a conspiracy theory about the 2016 U.S. election as well as Joe Biden and his son, Hunter, after the Ukrainian president thanked him for American military aid and asked for additional anti-tank missiles to combat Russia-backed separatists.

Bolton later advised Taylor to memorialize in a cable to Secretary of State Michael Pompeo his concerns about a White House decision to freeze military aid to Ukraine, the diplomat testified.

‘Hand Grenade’

Hill also recounted that Bolton on another occasion expressed serious concerns about Giuliani’s work in Ukraine. The former New York City mayor, then acting in his capacity as Trump’s personal lawyer, is “a hand grenade who’s going to blow everybody up,“ Hill recounted Bolton saying, according to the New York Times.

And Bolton allegedly asked Hill to report back to him on conversations between Sondland and Ukrainian officials -- including an instance when the ambassador specifically raised Burisma Holdings, the Ukrainian energy firm which counted Hunter Biden among its board members -- Hill said.

White House officials and allies of the president appear to have recognized the threat Bolton poses and have sought to neutralize it. Mulvaney said Sunday on Fox News that he was “surprised“ by Bolton’s apparent concerns, adding that the former national security adviser’s decision not to raise the matter with him directly was “a red flag.”

“John Bolton never complained to me about it,” Mulvaney said. “No one at NSC ever complained to me about anything that was going on.”

Sondland, who testified last week, said in his opening statement that if Bolton or others “harbored any misgivings about the propriety of what we were doing, they never shared those misgivings with me, then or later.”

And Giuliani told Fox News he was “disappointed“ in Bolton.

Bolton Silence

The president has adopted a more conciliatory tone in recent weeks, despite having blamed Bolton upon his ouster for “some very big mistakes” that “set us back” in global diplomacy. Trump told reporters on Oct. 16 that he “got along” with Bolton “pretty well,” while acknowledging there may have been disagreements between his former aide and Giuliani.

Bolton himself has so far refused to comment publicly on the Ukraine investigation. In a September address, he said that he had a “self-imposed restriction” on discussing specific details about his time in the administration.

Representative Gerald Connolly, a Virginia Democrat, said Monday that Bolton and Acting White House Chief of Staff Mick Mulvaney “should both be required to come before the committee.”

Bolton did not respond to a request for comment submitted to a spokeswoman at his political action committee.

But the former White House aide has signaled he’s willing to dish about his time in the West Wing. He has begun shopping a book about his tenure, Axios reported, and is represented by two literary agents who also sold tell-alls by former former FBI Director James Comey and former White House communications aide Cliff Sims.

He also re-launched his John Bolton PAC with an email to supporters on Thursday laced with thinly veiled criticism of Trump’s foreign policy, especially his outreach to North Korean leader Kim Jong Un. “Despite all the friendly notes and photo ops, North Korea isn’t our friend and never will be,” Bolton wrote.

But the same email showed why Democrats might be reticent to summon Bolton to testify. “Our nation’s commitment to our national security priorities is under attack from within,” he wrote, including through “impeachment” and “radicalized Democrats.”