John Bolton, former national security adviser. (Photo: Alex Edelman/Bloomberg)

Speculation is mounting over a potential court fight on former national security adviser John Bolton's testimony in a Senate impeachment trial—but what would a legal challenge to Bolton's upcoming book look like?

In a Jan. 23 letter obtained by several media outlets, Ellen Knight, the senior director for records, access and information security management at the National Security Council, told Bolton's attorney Charles Cooper of Cooper & Kirk that after a "preliminary review, the manuscript appears to contain significant amounts of classified information."

Knight wrote that some of the classified information is designated "top secret," which is defined as "reasonably could be expected to cause exceptionally grave harm to the national security" of the U.S.

"Under federal law and the nondisclosure agreement your client signed as a condition for gaining access to classified information, the manuscript may not be published or otherwise disclosed without the deletion of this classified information," the letter reads.

The manuscript of Bolton's upcoming book, "The Room Where It Happened," has found itself at the heart of the Senate's impeachment trial in recent days after The New York Times reported that it details conversations in which Trump directly linked Ukraine announcing investigations into former Vice President Joe Biden and his son Hunter Biden to lifting a hold on military aid to the country.

Senate Majority Leader Mitch McConnell, R-Kentucky, reportedly told other Senate Republicans on Tuesday he doesn't currently have the votes to block witnesses like Bolton from being called in the trial, but Republicans believe they'll be able to get needed support before the expected Senate vote on witnesses Friday.

National security lawyer Bradley Moss, who has previously challenged the government's requested redactions to book manuscripts in court, said in an interview that NSC's review of Bolton's manuscript could result in two possible outcomes.

In one, they can find the book is overwhelmed with classified information and the manuscript should be rewritten entirely. In the other scenario, which Moss said is more likely, the NSC can note which information is believed to be classified and request it be redacted or deleted from the manuscript.

Bolton can then file a First Amendment lawsuit to challenge the classification of the information. He would have to prove to a judge that the information labeled as being classified has already been released by the Trump administration, and therefore can be published.

"The only way to overcome it is to demonstrate the exact same information has been previously and officially disclosed by the U.S. government," Moss said.

The determination that classified information is in the manuscript also raises the possibility that Bolton could reveal that same information if called to testify in the Senate impeachment trial. If called to testify in the Senate impeachment trial, senators could follow the standard set during the Clinton impeachment and take Bolton's deposition off the Senate floor.

Some Republicans also have called for the White House to make the manuscript available to senators in one of the Sensitive Compartmented Information Facilities, or SCIFs, at the U.S. Capitol—a possibility that Democrats have slammed in favor of hearing from Bolton himself.

Bolton has already said he would comply with a subpoena for his testimony in the trial, after failing to appear for a requested deposition as part of the House's impeachment inquiry.

Moss was one of the lawyers behind a lawsuit during the Obama administration over a book from retired Lt. Col. Anthony Shaffer about his post-9/11 tours of duty in Afghanistan. American defense and intelligence agencies had requested hundreds of redactions from the book.

U.S. District Senior Judge Rosemary Collyer ruled in 2015 partially in Shaffer's favor, finding that parts of his book that was already revealed in congressional testimony could be published.

"With regard to all other redactions from the book, the court concludes that defendants have provided sufficient evidence that that the information redacted is properly classified and has not been officially released, and Lt. Col. Shaffer has failed to demonstrate a genuine issue of material fact otherwise," Collyer wrote at the time.

The government's prepublication review process is also being challenged in Maryland federal court by the ACLU and the Knight First Amendment Institute, on behalf of five former intelligence and military officials. DOJ lawyers filed a motion to dismiss; that motion has been fully briefed and is awaiting a ruling from the judge.

The Trump campaign also has accused former aides of violating nondisclosure agreements through arbitration actions. One such action was filed last year against former White House communications aide Cliff Sims over the publication of his tell-all book, "Team of Vipers."

Arbitrators have sided with the Trump campaign in upholding the agreements.

Sims sued Trump last year over the nondisclosure agreements, in a lawsuit filed by Moss and fellow national security lawyer Mark Zaid. The case was dismissed at the parties' request in early December, and Moss declined to comment.

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