WASHINGTON – John Bolton, a former Fox News commentator, had senior roles in three Republican administrations before beginning his tumultuous tenure as President Donald Trump's national security adviser.

Now, Bolton has a new title: Conservative target.

Since The New York Times reported that Bolton's soon-to-be-published book alleges Trump demanded Ukraine investigate Democrats in exchange for foreign aid, the reaction from the president and his allies has been fierce. They denounced Bolton as an opportunist, looking to cash in through a book airing unflattering details amid the Senate impeachment trial.

"I've repeatedly called him 'Book Deal Bolton,' " said Jason Miller, a former Trump campaign official and conservative podcast co-host.

People "feel betrayed by John Bolton," Miller said.

Trump slammed Bolton's book on Twitter Wednesday, calling it "nasty & untrue" and said that if he had listened to his former aide's national security advice, "we would be in World War Six by now."

Kelly backs Bolton:Former Trump chief of staff backs Bolton in Ukraine dispute

Some of Bolton’s associates downplayed the GOP rift over his forthcoming book, "The Room Where It Happened." Several predicted that even if Bolton were to testify in the Senate impeachment trial, he would try to limit the damage to his former boss.

But others who know the outspoken national security hawk insist that he is comfortable with controversy and will stand his ground. Bolton, they said, may be more concerned about his legacy than remaining loyal to Trump.

"I know John Bolton to be a man of integrity, true to his word and possessing an extraordinary memory," said Thomas Boyd, a friend and former Bolton deputy who said he could not speak to the accuracy of the Times' report on Bolton's book. "But I will say that John would never make something up on a matter of such importance."

'He's used to this'

Bolton's memoir alleges Trump told him he wanted to freeze nearly $400 million in Ukraine aid until officials in Kyiv opened an investigation into former Vice President Joe Biden, according to the Times. The revelation undermines a central Trump defense – that the money was withheld over concerns about corruption in Ukraine, rather than to pressure Kyiv to investigate the president's political rival.

The disclosure about the book has added to calls for witnesses at the impeachment trial and put new pressure on Republican senators.

While the White House and Trump's allies sought to marginalize Bolton, people who know the veteran of Washington knife fights likely expected the blowback.

Fred Fleitz, who worked for Bolton, called on his former boss to pull the book – at least until after the November presidential election.

In an interview with USA TODAY, Fleitz described Bolton as a man of principle who will stand up for what he believes.

"I think he's used to this," he said.

Book slammed as 'rancid, corrupt'

Fox Business Network host Lou Dobbs, a Trump ally, described Bolton's book as "a rancid, corrupt, absolutely disgusting move." Tucker Carlson, a Fox News host, called the manuscript a "betrayal." When Sen. Mitt Romney, R-Utah, said he would be open to calling Bolton as a witness in the impeachment trial, Sen. Kelly Loeffler, R-Ga., claimed in a tweet that her colleague wants "to appease the left."

Rep. Mark Meadows, R-N.C., a member of the president’s impeachment defense team, said the leak was a gift to Democrats.

“Some have suggested that it was to sell more books. Some have suggested that indeed the leak was designed to create chaos,” he said. “What we do know is that this is a pattern that my Democrat colleagues have embarked on when it was over in the House, and they continue to leak out things, trying to change it and change the narrative.”

Bolton's long-standing ties with conservatives have muddied the response and left some Republicans uncertain how to proceed. Democrats said Bolton's book should compel the Senate to hear from Bolton as a witness. Some Republicans, including Sen. Lindsey Graham, R-S.C., indicated they may be open to that idea.

Evidence or not:Trump lawyer says Bolton manuscript 'inadmissable'

Matt Schlapp, chairman of the American Conservative Union and another Trump ally, considers Bolton a friend. Bolton served on the group's board for years, Schlapp said, and was a regular speaker at its high-profile annual conference.

Schlapp questioned the accuracy of the leak and said he will withhold judgment until he can read Bolton's book himself. The memoir is scheduled to publish in March.

"I think John is wise enough to realize that doing anything in a political sense to help elect Bernie Sanders would be very regrettable," Schlapp said. "I don't believe that's his intention. When we hear from John Bolton, I believe he will reiterate his support for this president."

Senate reaction mixed

Most Senate Republicans have tread cautiously around the Bolton report, which Senate Majority Leader Mitch McConnell, R-Ky., acknowledged blindsided lawmakers.

Sen. Jim Inhofe, R-Okla., said he was “disappointed” by Bolton's book and his offer to testify but won't "say anything disparaging about it because he's a good friend for a long period of time."

When reporters asked Sen. Pat Toomey what he thought of Bolton – who has given heavily to his campaign – the Pennsylvania Republican hurried past them without responding.

Bolton, 71, established himself as a savvy Washington operator and aligned himself with the party's foreign policy hawks long before Trump entered the political scene. Trump tapped him to be his third national security adviser in 2018 after Bolton became a regular fixture on Fox News, espousing a get-tough view on Iran that aligned closely with the president's promise to unwind the Obama-era nuclear agreement with Tehran.

But Trump and Bolton read from different playbooks on North Korea and Russia, in particular, on which the president adopted a more conciliatory approach than his predecessors. Trump campaigned on withdrawing the United States from “endless wars,” while Bolton had long embraced a more aggressive, interventionist foreign policy agenda.

Those differences spilled into public view in September, when Trump said he fired Bolton but Bolton said he left on his own.

'Not out for revenge'

Some Bolton associates disagreed with the White House's characterization of his motives.

“John Bolton is not out for revenge,” said Mark Groombridge, who worked with Bolton for years at the State Department and outside government. “Bolton’s primary motivation is to advance his vision of national security.”

Groombridge predicted Bolton would limit his direct attacks on Trump.

“There’s no way his testimony or his book will cast the president in a favorable light,” Groombridge predicted. “I think he will portray him as undisciplined, impulsive, and he’ll chastise him for not listening to what Bolton considered sound advice."

If he provided the Senate with testimony that led to Trump’s removal from office, “he would be a pariah forever,” Groombridge said. That is unlikely to happen because "Bolton still wants to be a player in the GOP," he said.

John Yoo, a former colleague who has written several papers with Bolton, agreed that Bolton isn't driven by revenge.

"You’ll see people in the media suggest he’s settling scores. That is not John’s motivation,” said Yoo, a law professor at the University of California-Berkeley School of Law.

He said Bolton “is a believer in making sure history has a record of all the things he saw.”

Contributing: Christal Hayes, Nicholas Wu