WASHINGTON - For much of the last 20 years, John Bolton was a conservative poster child, a Republican hawk whose worldview helped shape the GOP establishment's approach to dicey foreign policy questions.

Now, as President Donald Trump's former national security adviser prepares to dish on his days in the White House, some old friends and colleagues are turning on him - and others are nervously wondering what he may be poised to reveal.

Leaked passages from the manuscript of Bolton's soon-to-be published book are roiling Washington, including the revelation that he says Trump told him he was conditioning the release of military aid to Ukraine on whether its government would help investigate former Vice President Joe Biden and his son.

Bolton's roughly 17-month tenure in as national security adviser brought him rare access to the inner workings of the West Wing and the Cabinet, particularly on matters of global affairs. He was known to take copious notes on yellow legal pads.

After this week's early leaks about the book, White House aides and allies are privately expressing concern about what more Bolton might reveal that the president and others in his orbit would find embarrassing.

Sen. Rand Paul, R-Ky., gave voice to GOP pique about Bolton's revelations in an interview with Fox News. He said Bolton had long argued for expansive executive powers to protect a president's conversations with his advisers but "now he's going to argue that no, no, no, now that I have a book deal for a couple of million bucks, that it's OK for me to say and spill the beans on everything the president's said to me privately.''

Less than two years ago, Rep. Mark Meadows, a North Carolina Republican and Trump impeachment team manager, praised the president for bringing Bolton into the White House, saying the president "could not have made a better pick'' for the key role in the administration. Bolton, who served as United Nations ambassador in the George W. Bush administration, was a fierce advocate for the 2003 war in Iraq and has pushed for military action in Iran, North Korea and Venezuela.

But this week, following news reports that Bolton made damaging charges about Trump in the forthcoming book, Meadows questioned why Democrats were treating Bolton as some sort of potential "super witness'' to make their case in the impeachment trial.

Sen. John Cornyn, R-Texas, earlier this month said Bolton's testimony "could well be beneficial'' to Trump in the impeachment trial. But after this week's leaks about the book, Cornyn questioned whether there would be much value in hearing from Bolton, saying "it's nothing different than what we've already heard.''

Democrats are pressing for Bolton to be called as a witness in the Senate trial, but need the support of at least four Republican senators to make it a reality. Only a few GOP senators have suggested they may be interested.

Fred Fleitz, a former chief of staff on Trump's National Security Council and a friend of Bolton, wrote in an opinion article for Fox's website that Bolton's decision to write a "tell-all'' was "crushing'' and urged him to withdraw it from his publisher immediately.

"I don't understand the need for a former National Security Adviser to publish a tell-all book critical of a president he served, especially during a presidential reelection campaign that will determine the fate of the country," Fleitz wrote. "There will be a time for Bolton to speak out without appearing to try to tip a presidential election.''

Trump himself has disputed Bolton's assertion of a concerted pressure campaign on Ukraine's leader. And Trump has dismissed Bolton as a disgruntled employee whom he fired because of policy differences. Bolton, who left the White House last year, insists he offered his resignation before Trump announced his ouster.

Trump jabbed at Bolton in a tweet Wednesday, referring to his former adviser as the "guy who couldn't get approved for the Ambassador to the U.N. years ago'' and accusing him of "mistakes of judgment.'' Bolton is well-known for his hard-line policy views against Iran and North Korea.

Bush installed Bolton as U.S. ambassador to the U.N. through what is known as a "recess appointment" when the Senate was out of session. He stepped down at the end of that appointment because he was unlikely to win Senate confirmation, largely due to his foreign policy vision.

In the tweet, Trump also complained that Bolton, after he was fired, "goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security. Who would do this?''

Trump allies, including personal attorney Rudy Giuliani and 2016 campaign adviser Jason Miller, slammed Bolton and said he was being untruthful in order to drive up book sales and remain relevant.

"There is no way in the world President Trump would say this to John Bolton," Giuliani tweeted. "It's a shame that a man will sacrifice his integrity to make a few bucks on a book. No wonder he accomplished so little as National Security Advisor."

In an interview scheduled for airing Wednesday on "CBS This Morning,'' Giuliani called Bolton a "backstabber.''

As Trump's legal team has presented the president's defense this week, it has made the case that Trump never conditioned aid on an investigation of the Bidens.

The president's lawyers have also argued that even if Trump had pressed for the investigation, it doesn't rise to level of an impeachable offense.

In a sideswipe at Bolton, one of Trump's attorneys, Jay Sekulow, argued Tuesday that senators cannot impeach a president on an "unsourced allegation.''

Administration officials have also sought to undercut another revelation from the Bolton manuscript, one reported Monday by The New York Times. It said Bolton told Attorney General William Barr last year that he had concerns that Trump was effectively granting personal favors to the leaders of Turkey and China. According to the manuscript, Barr told Bolton he was worried Trump was creating the appearance that he had undue influence over independent Justice Department probes.

The Justice Department has disputed the assertion that there was any discussion of personal favors or undue influence on investigations. The department also disputes that Barr ever said the president's conversations with foreign leaders were inappropriate.

"If this is truly what Mr. Bolton has written, then it seems he is attributing to Attorney General Barr his own current views - views with which Attorney General Barr does not agree,'' Justice Department spokeswoman Kerri Kupec said in a statement.

Others are standing by Bolton.

John Kelly, who served as Trump's chief of staff and overlapped with Bolton in the White House, said Monday in Florida that he believes the former national security adviser's account, the Sarasota Herald-Tribune reported.

"If John Bolton says that in the book, I believe John Bolton,'' Kelly said.