Sensational revelations from President Trump’s former national security adviser threatened to upend the Senate impeachment trial Monday, increasing the chances that senators would vote to allow witnesses in a perilous development for the White House.

Inside the Senate chamber as the trial entered its second week, Trump’s lawyers pushed forward with their defense of the president, largely ignoring the uproar caused by leaked details from a book by former national security adviser John Bolton. As part of their defense, they pivoted into a sharp line of attack on former vice president Joe Biden and his son Hunter over their involvement in Ukraine.

But outside the chamber, GOP senators caught unaware by the Bolton news grappled with divisions in their ranks and fresh calls from a small group of moderates who want to hear from Bolton before the third presidential impeachment trial in U.S. history comes to a close.

President Trump’s legal team resumed its defense presentation on Jan. 27 by criticizing the basis of the House’s impeachment inquiry. (The Washington Post)

Details that became public Sunday from Bolton’s unpublished book manuscript suggest that he could provide direct evidence that Trump sought to deny security assistance to Ukraine until Kyiv announced investigations into political opponents, including the Bidens. That linkage is at the heart of House Democrats’ case that Trump abused his power in holding up the Ukraine aid for his personal political benefit and then obstructed Congress’s subsequent investigation.

“I think it’s increasingly likely that other Republicans will join those of us who think we should hear from John Bolton,” Sen. Mitt Romney (R-Utah) said Monday, repeatedly calling Bolton’s testimony “relevant.” “It’s important to be able to hear from John Bolton, for us to be able to make an impartial judgment.”

Sen. Susan Collins (R-Maine), another key moderate, said the reports about Bolton’s unpublished manuscript “strengthen the case for witnesses and have prompted a number of conversations among my colleagues.” At least four Republicans would have to join with all Democrats for a vote to call witnesses to succeed, an outcome Majority Leader Mitch McConnell (R-Ky.) has sought to avoid because of the potential for the trial to devolve into a drawn-out mess.

Trump himself has vehemently denied any political motivation in stalling the Ukraine aid. He has insisted he never told Bolton that he was holding up the nearly $400 million in aid to force Kyiv to announce political investigations, suggesting that if Bolton said otherwise, it was only to sell books.

Trump’s attorneys devoted some of their floor time Monday to arguing that Trump acted appropriately in delaying the security aid, and that he did so because of his concerns about corruption in Ukraine and whether other nations were doing their fair share in providing security support for Ukraine.

The president’s defense team referenced the Bolton news obliquely shortly after the impeachment trial opened at 1 p.m., with Chief Justice John G. Roberts Jr. presiding and the sergeant-at-arms intoning his customary order for senators to stay silent “on pain of imprisonment.”

“We deal with transcript evidence, we deal with publicly available information,” said Trump attorney Jay Sekulow. “We do not deal with speculation, allegations that are not based on evidentiary standards at all.” A senior administration official, speaking on the condition of anonymity to discuss internal strategy, confirmed that Sekulow’s comments were a reference to the Bolton news.

Hours later, as constitutional-law professor Alan Dershowitz delivered the final arguments of the day, he briefly referenced the Bolton news as he argued that the charges against Trump do not ­constitute impeachable offenses. “Nothing in the Bolton revelations, even if true, would rise to the level of an abuse of power or an impeachable offense,” he said.

The signs of confusion among Senate Republicans over the Bolton revelations emerged early Monday at a news conference that was originally billed to have several Republicans but that dwindled to just Sens. Mike Braun (Ind.) and John Barrasso (Wyo.). Both senators said Trump remained on track to be acquitted, but while Barrasso dismissed the Bolton news as a “so-called blockbuster report” that contained “selective leaks,” Braun acknowledged its potency.

“I think what it does is, it’s taken an already hot topic and added some fuel to the fire,” Braun said.

Later, in a further sign of disunity, the newest Republican senator, Kelly Loeffler (Ga.), who was appointed last month, attacked Romney on Twitter. Loeffler, who has been a major donor and supporter for Romney in the past, contended Monday that he wanted to “appease the left by calling witnesses who will slander the @realDonaldTrump during their 15 minutes of fame.”

Much of the Republican lunch that preceded Monday’s trial proceedings centered on the question of witnesses, according to senators and other officials directly familiar with the meeting. The outcome of the witness vote in coming days could determine whether the trial is over by week’s end or extends into an uncertain future.

Romney spoke about the need to call additional witnesses, and Sen. Bill Cassidy (R-La.) also raised concerns. Cassidy noted that the White House has argued that there were no direct witnesses to any allegation of a quid pro quo, but that now a potential direct witness in the form of Bolton has emerged, according to an official with direct knowledge of the lunch who spoke on the condition of anonymity to describe the private discussion. Cassidy declined to comment about his remarks.

Democrats say calling witnesses is necessary for a complete trial, especially since Republicans have accused Democrats of relying on hearsay. In addition to Bolton, Democrats would like to hear from acting White House chief of staff Mick Mulvaney and several other officials.

“We want Bolton. We want Mulvaney. They heard from the president,” said Senate Minority Leader Charles E. Schumer (D-N.Y.).

One conservative senator, Patrick J. Toomey (R-Pa.), has spoken with Romney and other colleagues in recent days about possibly summoning two witnesses to the trial, one called by Republicans and one by Democrats, according to three Republican officials who spoke about the private discussions on the condition of anonymity. Toomey has argued that a “one for one” arrangement could force Democrats to accept a Republican witness against their wishes or else risk having Republicans move ahead to acquit Trump, the officials said.

However the question of witnesses is resolved, there remains little doubt that Trump will be acquitted in the end. Although it would take only a simple majority vote to call witnesses, a two-thirds vote is required for conviction.

As the debate over Bolton’s testimony and the potential need to call witnesses raged outside the Senate on Monday, Trump’s legal team inside the chamber delivered on its promise to turn its attention to Hunter Biden, and whether his role on the board of Ukrainian energy company Burisma gave the president good reason to ask his Ukrainian counterpart to investigate. Trump made that request in a July 25 phone call.

Pam Bondi, a former Florida attorney general who is a member of Trump’s defense team, laid out a detailed chronology of Hunter Biden’s five years on the board and how they dovetailed with the official actions of his father, then-Vice President Joe Biden, in Ukraine, as well as the campaign to oust former prosecutor general Viktor Shokin over corruption allegations.

Bondi relied heavily on illustrating the concurrent timeline of events, noting how many days transpired between Shokin’s ouster and the elder Biden’s announcements that the United States would provide security assistance to Ukraine.

She used several clips from mainstream newspapers and television networks about Hunter Biden’s Burisma connections to reinforce the idea that there was legitimate reason to scrutinize his hefty monthly paycheck, and played video clips from witnesses in the impeachment inquiry who testified that they found Hunter Biden’s role on the Burisma board troubling.

Joe Biden’s actions with regard to Shokin were in line with official U.S. and European policy objectives. And though Bondi provided no proof that Joe Biden’s actions vis-a-vis Ukraine were influenced by trying to advantage his son, that did not seem to be the ultimate goal of raising questions about the motivations behind the Bidens’ activities.

“All we are saying is that there was a basis to talk about this, to raise this issue,” Bondi said at the close of her presentation.

As Joe Biden’s name was invoked on the Senate floor, he was campaigning in Iowa, where Democratic voters will caucus in just a week’s time. Biden is a front-runner in the Democratic primary field, which Democrats say explains Trump’s desire for Ukraine to announce investigations targeting him and his family. A Biden campaign spokesman released a statement deriding Bondi’s remarks as a discredited “conspiracy theory.”

For members of the president’s legal team, it was the second day of their defense, after they began Saturday with an abbreviated two-hour summary. Each side is allowed 24 hours to present its case, and the House Democratic impeachment managers took nearly all their allotted time last week, repeatedly pushing the trial late into the night, to the irritation of some GOP senators. Trump’s team has promised to be more brief, although its presentation was continuing into the evening hours Monday and was expected to resume Tuesday.

Last week, Trump’s attorneys had to sit quietly and watch while the House Democratic managers delivered their arguments, but Monday the roles were reversed and the seven House managers sat quietly looking on as one member of Trump’s defense team after another rose to address different aspects of the proceedings.

The first Trump lawyer to speak at length was Kenneth Starr, the former independent counsel whose expansive investigation of Bill Clinton led to Clinton’s impeachment by the House in 1998 and subsequent acquittal by the Senate. Starr talked about the constitutional basis for the impeachment process, contending that it is at risk of being abused unless today’s Senate acts to stop it. Many Democrats viewed the presentation as the height of hypocrisy.

“We are living in what I think can aptly be described as the age of impeachment,” Starr said. “. . . Impeachment has now been normalized.”

Karoun Demirjian, Robert Costa, Rachael Bade, John Wagner, Elise Viebeck and Mike DeBonis contributed to this report.