US president Donald Trump’s legal team is nearing the end of his impeachment trial defence, painting him and his aides as hounded by investigation and taking a dismissive swipe at an unpublished book by John Bolton that is said to contradict a key defence argument.

“It is not a game of leaks and unsourced manuscripts,” said Mr Trump lawyer Jay Sekulow.

“That’s politics, unfortunately.”

He was referring to reports of a forthcoming book by Mr Trump’s former national security adviser, who writes that Mr Trump told him that he wanted to withhold military aid from Ukraine until it helped with investigations into Democratic rival Joe Biden.

Mr Trump and his lawyers have repeatedly insisted he never tied the security aid to political investigations.

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Personal lawyer to President Donald Trump, Jay Sekulow, speaks during the impeachment trial (Senate Television via AP)

While scoffing at the manuscript, Mr Trump and the Republicans have strongly resisted summoning Mr Bolton to testify in person about what he saw and heard as Mr Trump’s top national security adviser.

One of the president’s lawyers, Deputy White House Counsel Pat Philbin, told the senators that America’s Founding Fathers took care to make sure that impeachment was narrowly defined, with impeachable offences clearly enumerated.

“There has to be a defined offence in advance,” Mr Philbin said.

News of the Bolton manuscript clouded White House hopes for a big finish as well as a swift end to the impeachment trial.

Democrats are demanding witnesses, and some Republicans are expressing openness to the idea.

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Lindsey Graham believes the Bolton document should be made available to the Senate in a classified setting (Jose Luis Magana/AP)

One Republican, Senator James Lankford is floating an idea to subpoena Mr Bolton’s book manuscript so senators can see the evidence themselves — but only in private.

It is an idea that may be gaining traction even as other Republicans have warned against a protracted legal dispute with the White House, which has tried to block administration officials.

Republican Senator Lindsey Graham wrote on Twitter that he “totally” supports Mr Lankford’s proposal.

Mr Graham, a key Trump ally, said the Bolton document should be made available to the Senate, in a classified setting, “where each Senator has the opportunity to review the manuscript and make their own determination”.

However, Chuck Schumer, the Senate’s top Democrat, called the proposal, which would keep Mr Bolton out of public testimony, “absurd”.

“We’re not bargaining with them. We want four witnesses, and four sets of documents, then the truth will come out,” Mr Schumer said.

Senate Republicans were to meet behind closed doors to consider next steps.