Republicans ready to turn on Trump after Bolton book is leaked

His comments followed the leak of the manuscript of Mr Bolton's new book about his time in the White House.

"I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton," the Utah congressman said when he was asked if there are four Republican senators prepared to vote with the Senate's 47 Democrats on an expected measure to call the former White House national security adviser to give evidence.

"It's relevant. And, therefore, I'd like to hear it," Mr Romney said. "What impact that might have would be dependent on all the facts associated with it."

The former Republican presidential nominee would not predict how Mr Bolton's testimony might affect the final outcome of the trial.

Mr Romney's comment does not mean Democrats have the 51 votes they need just yet. But other Republican moderates who are vulnerable back home - Maine's Susan Collins, Colorado's Cory Gardner and Alaska's Lisa Murkowski - could join Mr Romney.

Then there is retiring Tennessee Senator Lamar Alexander, who is considered an institutionalist, and has not yet announced how he might vote on bringing in Mr Bolton and other potential witnesses.

There were signs that Republican senators are scrambling to react to a 'New York Times' (NYT) report that Mr Bolton's coming book - 'The Room Where It Happened' - will include his claim that Mr Trump once told him he wanted to continue holding up a $391m (€355m) military aid package meant for Ukraine until that country's new government announced investigations of top US Democrats, including the presidential candidate Jo Biden and his son Hunter.

Senate minority leader Chuck Schumer declared Mr Bolton's revelations were "stunning".

"It goes right to the heart of the charges against the president," the Democrat said.

"Ambassador Bolton essentially confirms the president committed the offences charged in the first article of impeachment," he said at a press conference, referring to the House of Representatives' abuse of power impeachment article.

Senator Chris Murphy said he thinks it might now be "impossible" for Republican senators to continue blocking witnesses during the trial.

During its opening hours, no Republicans voted with Democrats on several measures aimed at forcing the testimony of current and former Trump administration officials, as well as Democrats' attempts to obtain new documents related to the White House's Ukraine policies.

The book excerpt and leak could increase pressure on the Senate to call witnesses in the impeachment trial of the president and, beyond that, to challenge the core defence of Mr Trump, his lawyers and his supporters in Congress.

The 'NYT' reported that in his manuscript Mr Bolton writes of Mr Trump telling him in August "he wanted to continue freezing $391m in security assistance to Ukraine until officials there helped him with investigations" into Democrats including former vice president Mr Biden and his son Hunter Biden, who had been employed by a Ukrainian energy company.

That linkage is at the heart of the first article of impeachment against Mr Trump, which alleges the president put a condition on the aid to Ukraine and a visit to the White House by Ukrainian President Volodymyr Zelensky on the promise of an investigation that would serve to discredit a political rival.

Mr Bolton's reported claim, if true, would seriously undermine Mr Trump's attack on the reliability of the "abuse of power" charge.

His lawyers have argued claims of linkage between the aid and the investigations have come only from people with second or third-hand information.

Mr Bolton, as his book title suggests, says he was indeed in the room at a critical moment and that it did happen, according to the 'NYT' account.