FILE - National security adviser John Bolton speaks to reporters during the daily press briefing in the Brady press briefing room.
FILE - National security adviser John Bolton speaks to reporters during the daily press briefing in the Brady press briefing room at the White House in Washington, Nov. 27, 2018.

WHITE HOUSE - The Senate is to consider Friday whether to call witnesses in President Donald Trump's impeachment trial.

But it remains unclear whether enough Republicans will join the Democrats to compel such testimony. At least one senator of Trump's Republican Party is indicating that the revelations leaked from an upcoming book by former National Security Adviser John Bolton could persuade his colleagues.

The book undercuts one of the key points of the president's defense in his impeachment trial.
  
Senator Mitt Romney of Utah says Bolton has relevant testimony to provide, and he thinks "it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton."

FILE - Senator Angus King speaks at a Senate Armed Services hearing in Washington, April 11, 2019.

During an appearance on MSNBC, independent Senator Angus King of Maine predicted 10 or more Republicans will vote for documents and witnesses, perhaps defying Senate Majority Leader Mitch McConnell.

"So, I'll be surprised if the motion fails," King said. "The Republicans ought to build a statue of Mitch McConnell on the Mall, because that's party unity the likes of which is never seen."  

Other Republicans, speaking to reporters Monday, are holding firm.

"He's trying to sell a book," Senator John Kennedy, a Louisiana Republican, told reporters about Bolton. "I wouldn't bet my house on his credibility."  

Before Bolton spent 17 months as Trump's national security adviser, he had a long track record as a hawk on foreign policy, giving him credibility among many of the Republican senators who will have to decide if he should testify in the ongoing impeachment trial.  

The team around Trump quickly began turning against Bolton after The New York Times detailed that an excerpt from "The Room Where It Happened: A White House Memoir," to be published March 17, states Trump wanted to freeze military assistance to Ukraine until Kyiv's government announced an investigation into Democratic Party presidential contender Joe Biden and his son.  
  
The newspaper also reported that Bolton alleges after Trump's July 25, 2019, phone call with Ukrainian President Volodmyr Zelenskiy, the national security adviser raised his concern with Attorney General William Barr that the president's personal attorney, Rudy Giuliani, was pursuing a shadow Ukraine policy.
  
A senior legal adviser to the Trump re-election campaign, Jenna Ellis, who is also an attorney to the president, accuses Bolton of "willing to sell out America … just to score a book deal or five minutes of fame."
  
Retweeting Ellis on Monday, White House press secretary Stephanie Grisham said this was "So true & so unfortunate."

While Trump was hosting Israeli Prime Minister Benjamin Netanyahu at the White House on Monday, he was asked by a reporter about the key allegation in Bolton's book.
  
The president responded that "nothing was ever said to John Bolton" and that what is written in the book is "false."
    
Trump's defense team has maintained that he had valid reasons for withholding military aid from Ukraine.
  
In their arguments to senators, the president's lawyers are rebutting Democrats' allegation of a "quid pro quo." The Democrats say Trump was not going to help Kyiv until Zelenskiy announced an investigation of former U.S. Vice President Biden and his son, Hunter, who served on the board of Burisma, a Ukrainian energy company, while his father was in office.

Trump, early Monday morning, denied Bolton's account, saying on Twitter that his former adviser "never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book."

The president later inaccurately stated in a tweet that Democrats never asked Bolton to testify during last night's impeachment inquiry in the House.
  
Contents of Bolton's manuscript were submitted to the National Security Council for a standard security review on Dec. 30, 2019.
  
Bolton's attorney, Charles Cooper, is blaming the White House for disclosing contents of the book.    

WATCH: Possible Bolton testimony