John Bolton served as Mr Trump’s national security adviser until the President fired him last September, citing “strong” policy disagreements.

In that job, Mr Bolton interacted directly with Mr Trump during the events that led to his impeachment, which means he could provide the Senate with first-hand testimony about the President’s conduct - something currently missing from the case against him.

The Democrats want him to testify, along with other witnesses. The Republicans don’t, and they have the numbers to stop it.

RELATED: Trump fires national security adviser John Bolton

That is the context for today’s news.

Mr Bolton has been writing a book about his time in the White House, and according to The New York Times, drafts of the manuscript have been circulating in recent weeks.

That manuscript reportedly contains one particularly explosive claim.

Mr Bolton writes that, in August, Mr Trump told him he wanted to continue the freeze of $391 million in military aid for Ukraine until the country announced an investigation into his political opponent, Joe Biden.

If true, Mr Bolton’s revelation completely undermines one of the core planks of Mr Trump’s impeachment defence.

The President has repeatedly insisted there was no “quid pro quo” with Ukraine - that the aid freeze was not linked to his push for an investigation of Mr Biden, who is currently pursuing the Democratic presidential nomination and could be Mr Trump’s opponent at the election in November.

“Today’s explosive revelation that President Trump personally told former national security adviser John Bolton that he would continue the freeze on military aid to Ukraine until that country agreed to his political investigations confirms what we already know,” the Democrats’ House impeachment managers said in a statement reacting to the new revelation.

“There can be no doubt now that Mr Bolton directly contradicts the heart of the President’s defence, and therefore must be called as a witness at the impeachment trial.

“During our impeachment inquiry, the President blocked our request for Mr Bolton’s testimony. Now we see why.”

The statement called on senators to “insist” that Mr Bolton be called as a witness, and that he provide his notes and any other relevant documents.

“The Senate trial must seek the full truth and Mr Bolton has vital information to provide,” it said.

“There is no defensible reason to wait until his book is published, when the information he has to offer is critical to the most important decision senators must now make.”

Four Republican senators would need to side with the Democrats to compel witnesses to appear at the impeachment trial.

RELATED: John Bolton indicates he is willing to testify in Senate

Mr Bolton was already at the epicentre of the debate over witnesses.

In a letter to congressional investigators written in November, his lawyer claimed he knew about “many relevant meetings and conversations” which could shed new light on the President’s alleged efforts to pressure a foreign country, Ukraine, into announcing an investigation of his political opponent, Joe Biden.

Mr Bolton, he said, was “personally involved in many of the events, meetings and conversations about which you have already received testimony, as well as many relevant meetings and conversations that have not yet been discussed in the testimonies thus far”.

Despite that tease, Democrats in Congress ultimately decided not to subpoena Mr Bolton during their impeachment investigation, fearing a drawn out fight in the courts over his right to talk to them.

Mr Bolton would have been torn between two constitutional duties – on the one hand, to comply with the congressional subpoena, and on the other to follow a directive from the President not to appear.

By sidestepping that argument, the Democrats risked missing out on Mr Bolton’s testimony entirely. But in a statement released earlier this month, he announced he would appear at the Senate trial if called.

“During the present impeachment controversy, I have tried to meet my obligations both as a citizen and as former national security adviser,” Mr Bolton said.

“The House has concluded its constitutional responsibility by adopting articles of impeachment related to the Ukraine matter. It now falls to the Senate to fulfil its constitutional obligation to try impeachments, and it does not appear possible that a final judicial resolution of the still unanswered constitutional questions can be obtained before the Senate acts,” he said.

“Accordingly, since my testimony is once again at issue, I have had to resolve the serious competing issues as best I could, based on careful consideration and study. I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify.”

Whether the Senate ends up hearing from Mr Bolton or not, Mr Trump is unlikely to be removed from office. For that to happen, a two-thirds majority of the chamber would need to vote to get rid of him.

Put another way, at least 20 Republicans would have to vote against their own party’s President.

But the trial still has huge political ramifications, with the next presidential election now less than a year away.