Sen. Mitt Romney, R-Utah, said Monday it is more likely other Republicans will vote to hear witness testimony from former National Security Adviser John Bolton as part of impeachment proceedings following reports on new allegations in his forthcoming book -- as Sen. Susan Collins, R-Maine, added that the reports strengthened "the case for witnesses."

During a press gaggle in the basement of the Russell Senate Office Building, Romney said he thought Bolton's relevance to the impeachment trial was "apparent" in the wake of a New York Times report that his book claims President Trump connected frozen aid to Ukraine to investigations into the family of 2020 rival Joe Biden.

LEAKED BOLTON CLAIMS FUEL FIERY NEW CLASH OVER IMPEACHMENT TESTIMONY

"I think it's increasingly likely that other Republicans will ... join those of us who think we should hear from John Bolton and whether there are other witnesses and documents," he said. "I think it’s important to hear from John Bolton for us to make an impartial judgment."

Trump, meanwhile, denied the Bolton claims and told reporters Monday that they are “false.”

But Collins followed Romney's comments with a statement posted to Twitter just minutes later which indicated there may be other Republicans considering casting votes for Bolton as a witness.

"From the beginning, I've said that in fairness to both parties the decision on whether or not to call witnesses should be made after both the house managers and the President's attorneys have had the opportunity to present their cases," she said. "The reports about John Bolton's book strengthen the case for witnesses and have prompted a number of conversations among my colleagues."

Collins also reiterated that she will probably vote to call witnesses as she did during the Clinton impeachment trial.

Collins and Romney are the two GOP senators who have expressed the most interest in voting to hear witness testimony, particularly from Bolton.

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However, it remains unclear whether Democrats could convince another two Republicans to join them on such a vote, ensuring a majority to call witnesses.

"There is nothing new here to what House managers have been saying. ... Take a breath. Let's listen to the president's lawyers today," Sen. John Barrasso, R-Wyo., told reporters, referring to ongoing impeachment arguments in the Senate trial.

The attorney for Acting White House Chief of Staff Mick Mulvaney, who has also been eyed by Democrats for testimony, on Monday denied that he ever had a conversation indicating Ukraine aid was withheld in exchange for a Biden investigation.

"The latest story from the New York Times, coordinated with a book launch, has more to do with publicity than the truth," attorney Bob Driscoll said in a statement.

Sens. Lisa Murkowski, R-Alaska, Lamar Alexander, R-Tenn., and Cory Gardner, R-Colo., are among the other Republicans considered in play on a witness vote.

Murkowski said Monday she's still "curious" when asked about Bolton.

"I stated before that I was curious as to what John Bolton might have to say. From the outset, I’ve worked to ensure this trial would be fair and that members would have the opportunity to weigh in after its initial phase to determine if we need more information," she tweeted. "I’ve also said there is an appropriate time for us to evaluate whether we need additional information —that time is almost here. I look forward to the White House wrapping up presentation of its case."

The moderate Murkowski has remained noncommittal on how she would vote. Gardner, who has a reputation for not being afraid to buck Trump and is running for reelection in a purple state, has remained largely mum on his thoughts about the impeachment trial. Alexander, a generally reliable GOP vote – but a Senate institutionalist – has said he would make up his mind after the conclusion of opening statements and questions.

Democrats, meanwhile, used the Bolton reports to apply immense pressure to GOP senators and the White House.

"We're all staring a White House cover-up in the face. It is so clear what's going on here," Senate Minority Leader Chuck Schumer, D-N.Y., said.

Fox News' Jason Donner and Lillian LeCroy contributed to this report.