Former White House Press Secretary Ari Fleischer said Monday that former national security adviser John Bolton “has a moral obligation to step forward and explain what he knows” after a report in The New York Times revealed a bombshell excerpt from Bolton's forthcoming book that could prove pivotal in President Trump's impeachment trial.

Speaking on “America’s Newsroom,” Fleischer, a Fox News contributor, acknowledged that if Bolton explained what he knows it “might hurt his book sales,” adding that he still has a “moral obligation” to speak with the media.

“If this has been interjected now into an impeachment trial in the Senate and is considered by some [as] such important evidence, and he [Bolton] might not be able to be subpoenaed because executive privilege is a legitimate reason not to be subpoenaed, then he has a moral obligation to go to the press and tell the press what he knows,” Fleischer told anchor Ed Henry.

“He cannot let this half-leak stand,” he continued. “If there's context to it, John should explain it. If there is no context to it, John should state so. You don't just let this linger.”

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The excerpt was published in The New York Times on Sunday. The newspaper exclusively reported that Bolton's manuscript included a claim that Trump explicitly linked a hold on Ukraine aid to an investigation of Joe and Hunter Biden. According to a transcript of Bolton's forthcoming book reviewed by the Times, Trump told Bolton in August "that he wanted to continue freezing $391 million in security assistance to Ukraine until officials there helped with investigations into Democrats including the Bidens."

President Trump refuted Bolton's claims on Twitter on Sunday.

Reacting to the leak Fleischer said on Monday, “My big question here is, what else is in that book? Is there information in there that puts it in a different context? Is there information where the president said to John Bolton, ‘I want to hold up aid because of burden sharing, I am not sure I want to proceed with the aid until our allies kick in more, or until something is done about corruption.’"

"This obviously was put out by somebody who is opposed to the president's position here and it was timed as such,” he continued.

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“Having said that, it still is problematic. One way or another we are going to hear from John Bolton. I suspect it won't be until his book comes out though.”

In the coming days, the Senate is expected to vote on whether Trump’s trial will permit any witnesses. Democrats want to hear from witnesses including Bolton, who said earlier this month that he would testify should he be subpoenaed by the Senate.

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Speaking at a news conference in Switzerland last week, Trump said Bolton‘s potential testimony could pose "a national security problem.” He indicated that he would assert executive privilege to prevent Bolton from testifying in the Senate impeachment trial.

Fox News’ Gregg Re and Gillian Turner contributed to this report.