Bolton, who has served under Republican presidents since Ronald Reagan, again showed his deftness at the Washington game Wednesday when multiple newspapers published some of his juiciest allegations - just as the Trump administration was trying to block the book, saying it contained classified material.

But his motivations - as well as his writing style - were widely called into question.

In a stinging review, New York Times book critic Jennifer Szalai scoffed at Bolton's explanations for why he did not testify in Trump's impeachment.

"The clotted prose, the garbled argument and the sanctimonious defensiveness would seem to indicate some sort of ambivalence on his part - a feeling that he doesn't seem to have very often," she wrote.

She said that Bolton described his turning point as when Trump called off plans to strike Iran, even after the national security advisor went home for a change of clothes, excited for a long night at the White House.

Many of the early excerpts criticise Trump over his wooing of North Korean strongman Kim Jong Un - on which Bolton's opposition was well known, contributing to his exit in September.

Bolton charges in the book that Trump is concerned only about winning re-election and not any larger US interest and makes the bombshell allegation that the president asked his Chinese counterpart Xi Jinping for help securing a second term.

Both Democrats and Republicans questioned why Bolton, if he felt so strongly about Trump's conduct, waited to speak now.

The liberal television commentator Lawrence O'Donnell said that Bolton, "like Trump, is a dishonorable man who cares more about money than his constitutional duty."

Republican Representative Doug Collins pointed to Bolton's reported $2 million advance for the book, tweeting: "Apparently that's all it took for John Bolton to sell off what little credibility he had left."

But Bill Kristol, a prominent conservative thinker who opposes Trump, wrote on his website The Bulwark that "almost every Republican official" knows Bolton personally and no one considers him a liar.

"John Bolton may not be the epitome of warmth, humor or even kindness. But he is honest," Kristol said.