John Bolton has long been known as the bomb-thrower of US diplomacy, but the former White House national security advisor now threatens to blow a hole in President Donald Trump’s impeachment defense.

Fired last September, Bolton’s coming tell-all book about his 17 months in the Trump administration could contain the strongest evidence yet for charges that the president illicitly held up military aid to Ukraine to help his own reelection campaign.

A draft of “The Room Where it Happened,” scheduled for publication in March, reportedly says Trump told Bolton last August the money -- almost $400 million -- would remain frozen until Kiev announced an investigation into Trump’s potential election rival, Democrat Joe Biden.

That alleged “quid pro quo” is at the heart of the impeachment charge of abuse of power that has Trump on trial in the Senate, facing possible removal from office.

Until The New York Times report Sunday of the book’s claim, Democrats had faced accusations that they had only indirect evidence of a link between Trump withholding support to Ukraine and pressing the US ally to announce a Biden probe.

It had appeared certain that the Republican-dominated Senate would vote easily to acquit Trump.

But the leak of the manuscript now challenges the senators to call Bolton to testify against the president -- with unpredictable consequences.

Faced with the prospect of a Bolton appearance, Trump has made his position clear.

“I don’t know if we left on the best of terms. I would say probably not, you know. And so, you don’t like people testifying when they didn’t leave on good terms,” the US President admitted candidly last week.

The diplomatic world has long been accustomed to Bolton’s incendiary presence and his trademark bushy moustache.

An unabashed hawk, he served in the Justice and State Departments of three Republican administrations, starting with Ronald Reagan’s in the 1980s, before joining Trump as national security advisor in April 2018.

In the 2000s he made his mark on the world stage as US ambassador to the United Nations (UN), where he openly disdained the institution and unapologetically heralded US global projections of its power.

That hard line never sat entirely comfortable with the conservative presidents he served, but he relished his role as a provocateur and successive administrations used him as an effective “bad cop” whose threats could strengthen the US line in negotiations.

True to form, last year while the Trump administration was seeking a negotiated settlement to the political stand-off in Venezuela, Bolton openly suggested US military intervention.

“He has strong views on things but that’s okay. I actually temper John, which is pretty amazing,” Trump said in May 2019.

But Bolton’s strident opposition to Trump negotiating with Iran, North Korea and the Taliban eventually forced his ouster -- though he insists he resigned -- last September.

In testimony late last year other national security council officials told the House impeachment investigation Bolton staunchly opposed Trump’s pressuring Ukrainian President Volodymyr Zelensky to announce investigations into the Democrats.

Bolton told aides to stay away from the Ukraine affair and report it to White House lawyers, characterizing the actions of Trump’s inner circle as a “drug deal.” But he rebuffed a request to turn over documents and testify to the House investigation after Trump ordered current and former staff not to cooperate or hand over any records.

Rather than subpoena him, which could have resulted in a year-long court battle, the House sent the impeachment case against Trump to the Senate for trial without his evidence.

Since then Bolton has been cagey. He has not denied the testimony of his former aides, but he has refused to speak in public about the Ukraine affair, instead hinting his book would tell all.

At the same time, he issued a statement saying that if subpoenaed by the Senate trial, he was prepared to testify.

That, however, will require two things: that at least four Republicans, who hold a 53-47 majority in the Senate, agree with Democrats to issue the subpoena, and secondly, for Trump to not block the testimony with a claim of executive privilege.

That could mire the impeachment trial in a court fight over presidential powers -- while significantly raising expectations for the release of Bolton’s book.