President Donald Trump and his allies have turned against former national security adviser John Bolton, portraying him as part of a "deep state" determined to unseat the president.

The attacks came in response to reports that Bolton's upcoming memoir undermines a central part of Trump's impeachment defense.

The book said Trump made Ukraine dependent on a probe being launched into his Democratic opponent, former Vice President Joe Biden.

Fox Business host Lou Dobbs, who often advises Trump, said Monday: "John Bolton himself has been reduced to a tool for the radical Dems and the deep state" with the allegations in his book.

Bolton, a longtime advocate of the US use of military force abroad, has long been a darling of the conservative right.

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John Bolton has long been lauded by the Republican right. The hawkish foreign-policy adviser, who has served four Republican presidencies, used to be a mainstay on Fox News.

But with revelations from his forthcoming memoir undercutting a key part of President Donald Trump's impeachment defense, and roiling the president's Senate trial, allies of Trump are turning against the former national security adviser.

The New York Times reported on Sunday that Bolton wrote in his book that Trump told him last year that he would withhold military aid to Ukraine until Ukrainian President Volodymyr Zelensky acceded to his demands to investigate his political rival, Democratic frontrunner Joe Biden.

Such testimony would be the strongest evidence yet linking Trump to a so-called quid pro quo of congressionally-approved military aid in exchange for investigations that could benefit Trump politically.

'A tool for the radical Dems'

Bolton's old colleagues at Fox News — where he worked as a contributor — are now gradually turning against him.

In his Monday show Lou Dobbs — host of Fox Business and informal adviser to the president — went so far as to accuse Bolton being a "tool for the radical Dems," unveiling a complex chart which he claimed exposes damning links between Bolton, Republican opposed to Trump, and former FBI Director James Comey.

"Not so complicated, is it?" Dobbs said. "John Bolton himself has been reduced to a tool for the radical Dems and the deep state with his, well, with his allegation that the president once told him the aid to the Ukraine was entirely dependent on whether or not Mr. Zelensky carried out investigations of his political opponents."

The segment went viral among Trump supporters online, and Trump on Monday shared a tweet by Dobbs that described Bolton's claims as the "deep state's last desperate act."

'You have something to say, John, come here'

Dobbs wasn't the only Fox host who devoted a portion of their show to denouncing Bolton.

On Tuesday, Sean Hannity demanded angrily that Bolton come on his show to explain himself after the politically damaging portions of his memoir leaked.

"John Bolton, I say to you tonight, you have something to say, John, come here," the Fox News host exclaimed. "You worked here. This is your old home. Come on the show. Have your say on this show. We've invited you repeatedly. Radio and TV, over the past number of weeks."

In a series of Wednesday-morning tweets, Trump called Bolton's upcoming memoir "nasty & untrue" and claimed that had he listened to Bolton, "we would be in World War Six by now."

Just another page out of the Trump playbook

On Tuesday, The Washington Post reported that a network of pro-Trump bloggers and conservative websites were dredging negative stories from Bolton's past, and seeking to portray him as a traitor to the president.

The coverage prepared the ground for attacks by Fox hosts, who later cited those stories, The Post reported.

Smearing opponents as part of an establishment conspiracy is a familiar tactic from the Trump playbook, and a line of attack the president and his allies have taken against opponents as varied as 2016 Democratic nominee Hillary Clinton, disgruntled former intelligence officials critical of Trump, and Republican Sen. Mitt Romney.

From left to right: Then-Chief of Staff John Kelly, Trump, and Bolton. Leon Neal/Getty Images

Associates of Bolton — who has long been reviled by liberals — have expressed their surprise at him joining their ranks.

"It shows you once again that there's very little honesty, decency or civility in Trump and the crowd around him," the Republican former defense secretary Chuck Hagel told The Post.

"I had my differences with him, but I would say that John Bolton is anything but 'deep state.'"

Trump's former chief of staff, John Kelly, also told an audience in Florida on Monday night: "If John Bolton says that in the book, I believe John Bolton."