Republican senator Mitt Romney says it is "increasingly likely" that four of his colleagues will support the calling of ex-Trump adviser John Bolton to testify in the president's impeachment trial – after a manuscript of his new book about his time in the White House was leaked.

"I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton," the Utah politician told reporters on Monday when asked if there are four GOP senators prepared to vote with the chamber's 47 Democrats later this week on an expected measure on calling the former White House national security adviser to testify.

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

The former GOP 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 GOP 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 GOP Sen Lamar Alexander, who is considered an institutionalist, and has not yet announced how he might vote on bringing in Bolton and other potential witnesses.

All the president's lawyers: The team fighting Trump's impeachment Show all 6 1 /6 All the president's lawyers: The team fighting Trump's impeachment All the president's lawyers: The team fighting Trump's impeachment Alan Dershowitz Dershowitz is a controversial American lawyer best known for the high-profile clients he has successfully defended. Those clients have included OJ Simpson, Jeffrey Epstein and Harvey Weinstein. One longtime Harvard Law associated told the New Yorker Dershowitz "revels in taking positions that ultimately are not just controversial but pretty close to indefensible." Getty All the president's lawyers: The team fighting Trump's impeachment Ken Starr Starr became a household name in the 1990s as the independent counsel who led the investigation that led to Bill Clinton's impeachment. That investigation began as a look into a real estate scandal known as Whitewater, and eventually led to impeachment after Mr Clinton lied under oath about having an affair with White House intern Monica Lewinsky. AP All the president's lawyers: The team fighting Trump's impeachment Jay Sekulow Sekulow is the president's longtime personal attorney, and, now, personal lawyer in the White House. He has been accused by former Rudy Giuliani associate Lev Parnas of being "in the loop" during the Ukraine scandal. Getty All the president's lawyers: The team fighting Trump's impeachment Pam Bondi Bondi is the former attorney general in Florida, and a longtime backer of the president's. She made a name for herself in Florida for taking hyper partisan stances on issues, and her penchant for publicity. She is likely to be a prominent public-facing figure during the trial. AFP/Getty All the president's lawyers: The team fighting Trump's impeachment Pat Cipollone Cipollone is the White House counsel, and leading the president's defence team. Getty All the president's lawyers: The team fighting Trump's impeachment Rudy Giuliani While not officially named as one of the president's impeachment lawyers, it is hard to ignore Giuliani's outsized role in this process. The former mayor of New York has been making headlines for months as he defends his client, and for his apparent role in the effort to compel Ukraine to launch the investigation into Joe Biden. We'll see how he figures in the actual trial, which he has said he would like to be a part of. Reuters

Ms Collins issued a statement noting she has said she likely would support calling witnesses in a vote expected this week. But she did not confirm there are two other GOP senators who have privately said they intend to do the same, saying: "The reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues."

There were signs on Monday morning that GOP senators were scrambling to react to a Sunday evening New York Times report that Mr Bolton's coming book will include his claim that Donald Trump once told him he wanted to continue holding up a $391m military aid package meant for Ukraine until that country's new government announced investigations of top US Democrats, including the Bidens.

For instance, Senate Judiciary Chairman Lindsey Graham of South Carolina cancelled a planned press conference.

More on Republicans' response to the Bolton bombshell could come soon. GOP senators are scheduled to huddle later on Monday, before the chamber resumes the impeachment trial. Mr Trump's legal team will continue laying our their case in defence of the president.

Senate Minority Leader Chuck Schumer called Bolton's revelations "stunning", but said he still sees an "uphill fight" to get the four GOP senators he needs because of "the pressure from the White House" on the entire Republican caucus.

"It goes right to the heart of the charges against the president. Ambassador Bolton essentially confirms the president committed the offences charged in the first article of impeachment," the New York Democrat told reporters during a late-morning press conference, referring to the House-passed abuse of power impeachment article.

Mr Bolton said recently he would testify if called by the Senate, but Mr Trump told reporters last week that would create a "national security problem". Should enough GOP senators join Democrats in supporting Mr Bolton's appearance during the trial, the White House has threatened invoking executive privilege, meaning a court fight could bog down the trial's pace.

Mr Schumer called the former Trump aide "ready and willing to testify", pressuring those across the aisle by saying: "How can Senate Republicans not vote to call that witness, and request his documents? Anyone who says the House case lacks eye witnesses, but then votes to block eye witnesses, is talking out of both sides of their mouth."

The Senate Democratic leader accused top White House leaders of orchestrating a "giant cover up" because some officials at the executive mansion had a copy of Mr Bolton's manuscript months ago as part of a standard security review of its contents.

"It is so clear what's going on here. ... If Senate Republicans are not going to vote to call Mr Bolton and Mr Mulvaney and the other witnesses now, if they're not going to ask for notes and emails, they're going to be part of the cover up, too, because we have this out in the open," Mr Schumer said, referring to Acting White House Chief of Staff Mick Mulvaney, who current and former administration officials told House investigators was heavily involved in the Ukraine affair, which Mr Bolton once dubbed "a drug deal".

The minority leader seemed eager for a court fight, calling any possible coming subpoena for Mr Bolton "bipartisan by definition" because at least four Republicans would vote for anything that might eventually pass. He also noted it would be signed by Supreme Court Chief Justice John Roberts, who is presiding over the Senate trial. Along with those things, Mr Schumer noted the Constitution gives the Senate the authority to run an impeachment trial. Collectively, he predicted, the courts would waive any executive privilege claim – a dare to the White House that just might be met with a court battle.

Connecticut 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 GOP lawmakers 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.

GOP senator Mike Braun followed Mr Schumer in the same studio, flanked by GOP senator John Barrasso. The latter called the NewYork Times report a "so-called blockbuster" article, and sought to discredit Mr Bolton's account by saying it adds no new information to the trial. Notably, however, the Wyoming Republican did not repeat Mr Trump's denial that he ever said he wanted to keep the military aid package frozen until Ukraine announced the investigations of the Bidens and other Democrats.

"I would say, at this point, take a breath, listen to what we hear from the president's defence team, listen to the questions [from senators] this week," Mr Barrasso said after noting the Iowa Caucuses are on Monday. "And let the American people decide who should be their president."