Republican Senator Mitt Romney has said it is "increasingly likely" that at least four Republican Senators will join Democrats to give them the majority of votes needed to call former National Security Advisor John Bolton to testify in the Senate impeachment trial of United States President Donald Trump.

The statement comes a day after The New York Times reported that Trump had told Bolton in an August conversation that the president wanted to continue to withhold congressionally approved military aid from Ukraine until officials from the country agreed to help with investigations of his Democratic political rival Joe Biden and his son, Hunter Biden.

Bolton had detailed the exchange in a draft of his new book, which he circulated to a small group of confidants and submitted for standard review to the White House, according to the newspaper. Trump has denied making the statement to Bolton.

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"I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton," Romney, who represents Utah, told reporters on Monday.

Meanwhile, Republican Senator Susan Collins of Maine said the case for calling witnesses had been strengthened by the Times report.

"The reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues," Collins said in a statement she posted to Twitter on Monday.

My statement on Bolton developments. pic.twitter.com/3M59J7suts — Sen. Susan Collins (@SenatorCollins) January 27, 2020

In December, the House of Representatives voted, mostly along party lines, to impeach Trump for abuse of power and obstruction of Congress. For the first charge, investigators allege Trump used his office to withhold nearly $400m in military aid in exchange for Ukraine's commitment to investigate Former Vice President Joe Biden's son Hunter's involvement with Ukrainian gas company Burisma.

Joe Biden is a frontrunner in the 2020 presidential Democratic race. There is no evidence of wrongdoing by the Bidens.

Trump had repeatedly denied the impeachment allegations, branding the proceedings a "hoax".

Bolton, who was the national security adviser when the aid was withheld, has said he would testify in the Senate trial if subpoenaed. Bolton was forced out of the White House last year following a number of disagreements with Trump.

The White House has categorically blocked administration personnel from testifying in the inquiry, although several staff members have appeared before House committees after being subpoenaed.

Debate over witnesses

The Senate trial will enter its seventh day on Monday, with Trump's defence team beginning its second of three allocated days of arguments. Defence team members, like the Democratic House managers prosecuting the case last week, will have a total of 24 hours to make their case.

Under Senate Majority Leader Mitch McConnell's rules resolution passed at the beginning of the trial, debate and a vote over whether witnesses and other new evidence will be allowed in the trial will come after the defence team's arguments conclude and after Senators are given 16 hours to pose questions to both the prosecution and the defence.

Republicans make up 53 of the 100 seats in the Senate and a simple majority is needed to call new witnesses. The makeup of the chamber has focused attention on just a few Republicans who are considered open to breaking from the party ranks and voting with Democrats. They include Collins, Romney, Lisa Murkowski of Alaska, and Lamar Alexander of Tennessee, among others.

To date, Romney is the only Republican senator who has publicly said he would vote in favour of hearing Bolton's testimony.

'Hand grenade'

Bolton came up several times during the hours of testimony in the House's inquiry, with his aide, Fiona Hill, describing several colourful exchanges with her former boss regarding Trump's alleged pressure campaign.

Bolton described Trump's personal lawyer Rudy Giuliani, who allegedly lead shadow efforts in Ukraine on the president's behalf, as "a hand grenade who's going to blow everyone up", according to Hill.

Hill also recounted a meeting in which Bolton "stiffened" when US Ambassador to the European Union Gordon Sondland brought up the Ukrainian investigations sought by Trump.

According to her testimony, Bolton, the top adviser on national security matters in the White House, later told Hill to report the incident to the White House lawyer.

Referencing White House Chief of Staff Mick Mulvaney, Bolton told Hill to tell the lawyer "I'm not part of whatever drug deal Sondland and Mulvaney are cooking up", the former aide testified.