Washington, DC - The defence team of United States President Donald Trump is set to deliver its last day of opening arguments on Tuesday as it attempts to brush aside new revelations from the former White House national security adviser, John Bolton, and growing pressure for witnesses.

"If a president, any president, were to have done what the Times reported about the content of the Bolton manuscript, that would not constitute an impeachable offence," Trump defence lawyer Alan Dershowitz said on Monday night. He was referring to a forthcoming book by Bolton in which the former White House aide reportedly writes that Trump told him he wanted to continue to withhold $391m of congressionally-approved military aid from Ukraine until the Eastern European country agreed to help with investigations of the president's Democratic political rival Joe Biden, and his son Hunter.

"Let me repeat: Nothing in the Bolton revelations - even if true - would rise to the level of an abuse of power or an impeachable offence," Dershowitz said late on Tuesday, after the defence team largely ignored the reports about Bolton's book during its opening arguments.

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Outside the Senate chamber, however, the revelations prompted calls and debate among some Republicans to consider calling Bolton as a witness.

"It's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton," Republican Senator Mitt Romney, who favours calling witnesses, told reporters in the Capitol.

Republican Senator Susan Collins issued a statement that "the reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues".

"What it's done is take an already hot topic and added fuel to the fire," said Senator Mike Braun, a Republican who wants to acquit Trump of the House impeachment charges of abuse of power and obstruction of Congress.

Trump was impeached in December for abuse of power and obstruction of Congress. He has denied any wrongdoing.

Trump's defence lawyers have argued that there is no direct evidence that the president conditioned aid for Ukraine on political investigations of the Bidens.

But following a New York Times newspaper report about what's in the book, Democrats say Bolton can provide first-hand knowledge of Trump's pressure campaign.

"The case for witnesses and documents was made even stronger by the revelations in the New York Times about Mr Bolton. It cries out for witnesses and documents," Democratic Senate leader Chuck Schumer told reporters.

"Every few days there's another revelation and another revelation and another revelation, and the case gets stronger and stronger," Schumer said.

Schumer speaks during a news conference about the impeachment trial of Trump [J Scott Applewhite/AP Photo]

Senator Angus King, an independent, told National Public Radio, he was "already hearing a number of Republicans are moving toward voting to at least vote to hear from John Bolton".

"I've known John Bolton. He's a person of integrity and he'll be honest," Republican Senator Ron Johnson told Al Jazeera.

"But even within that people have different perspectives, even within the conversation. People have different memories," Johnson said.

Trump has denied Bolton's account, tweeting: "I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens. If John Bolton said this, it was only to sell a book."

Bolton has not spoken publicly about the manuscript.

Trump team focuses on the Bidens

Trump's lawyers, meanwhile, detailed the timeline around a potential conflict of interest in Hunter Biden's acceptance of a paid position on the board of Burisma, a Ukrainian energy company, while his father was vice president.

Hunter Biden was paid an estimated $3m over three years to be on the Burisma board, Trump's defence claims.

"All we are saying is that there was a basis" for President Trump "to raise the issue" with Ukraine President Zelenskyy, argued Pam Bondi, a former Florida attorney general and member of Trump's team.

In this image from video, Alan Dershowitz, a lawyer for President Donald Trump, speaks during the impeachment trial [Senate TV/AP Photo]

There has been no evidence of wrongdoing by the Bidens.

Following the end of opening arguments, senators will have 16 hours for written questions. A vote on witnesses is expected as soon as Friday.

A two-thirds majority vote in the Senate would be required to convict and remove Trump from office, an unlikely outcome. Republicans hold a 53-47 majority.