President Donald Trump's former national security adviser has upended the Senate impeachment trial, and new revelations from John Bolton's draft book manuscript could turn the tide on whether senators call for witnesses.

The President's legal team resumed its second day of arguments just after 1 p.m. ET Monday, but much of the attention on Capitol Hill focused on the Republican senators and how they are reacting to Sunday night's New York Times bombshell that Bolton's draft manuscript says Trump told him US security assistance to Ukraine was conditioned on investigations into Democrats, including former Vice President Joe Biden.

Two key moderate Republicans said the Bolton news strengthened the case for having witnesses in the trial — and Republican Sen. Mitt Romney of Utah predicted it was "increasingly likely" that other Republicans would now join him in calling for Bolton to testify.

But other GOP senators, including in Republican leadership, downplayed or dismissed the developments.

Jay Sekulow, the President's private counsel, alluded to the Bolton allegations during his opening remarks but suggested it would not be discussed by the team on Monday.

"What we've done on Saturday is the pattern that we're going to continue today as far as how we're going to deal with the case. We deal with transcript evidence, we deal with publicly available information. We do not deal with speculation, allegations that are not based on evidentiary standards at all," he said.

Hours into the trial, Bolton's manuscript has not been directly addressed.

Sekulow said the team, including Judge Kenneth Starr, who led the investigation leading to President Bill Clinton's impeachment, would lay out an overview of "historical and constitutional issues with impeachment proceedings."

Democrats ratcheted up the pressure on the Senate to call for witnesses in response to the draft book manuscript, while the President's legal team debated how to address it during their argument on the Senate floor on Monday.

It's a conversation that shifted far from where the President's lawyers and Republican senators thought they were going this week when the President's trial resumed. Republican sources thought Saturday they were confident that they had the votes to defeat a motion for additional witnesses and documents, leading to an acquittal vote by the end of the week.

Now that's all in doubt.

"I can't begin to tell you how John Bolton's testimony would ultimately play on a final decision but it's relevant," Romney told reporters Monday. "And therefore, I'd like to hear it."

Sen. Susan Collins of Maine, a moderate Republican, said in a statement that the reports on Bolton's book "strengthen the case for witnesses and have prompted a number of conversations among my colleagues."

Republicans mixed on Bolton allegation

Republican senators were peppered with questions about Bolton as they returned to the Capitol on Monday for the resumption of the impeachment trial, which paused Saturday after the defense counsel presented for two hours.

While Romney and Collins said the Bolton news strengthened the argument for calling him, other Republicans downplayed the need for Bolton be a witness or argued that it wasn't relevant to deciding whether to convict the President on the two articles of impeachment.

GOP sources expect the Senate Republican leadership to reiterate to their conference the arguments they've been making for weeks: That seeking Bolton testimony would raise constitutional and executive privilege concerns — and argue that going through a protracted legal fight for his testimony would accomplish very little since Trump is expected to be acquitted anyway. One GOP aide told CNN Monday morning that Bolton news doesn't change the Republicans' underlying point — if you aren't going to vote to remove him, why drag the process out with witnesses?

Plus, they will reiterate that it was the House's job to conduct the investigation, not the Senate's.

Democrats seized on the new Bolton allegations, arguing that anyone seeking to learn the truth should want to hear from the President's former national security adviser in light of the revelations.

"It completely blasts another hole in the President's defense," said House Intelligence Chairman Adam Schiff.

Schiff, the lead House impeachment manager, said the Bolton news "makes it all the more clear why you can't have a meaningful trial without witnesses and you certainly can't have one without one without John Bolton."

Trump team debates how to handle Bolton

The President denied Bolton's allegations on Twitter early Monday morning, and continued to attack the impeachment trial and the managers as the trial was set to resume.

In the wake of the Bolton news, the President's legal team wrestled with whether to address the manuscript directly on the Senate floor, according to a person briefed on the discussions. It became a major source of debate, with some advising it should be ignored while others said there's no way they couldn't talk about it.

In the hours after the Times broke the Bolton story, Republican senators began reaching out to the White House to control the damage. Both Graham as well as McConnell were in communication and there was an expressed view that the GOP Senate should have been kept in the loop.

One message conveyed to the White House, according to a GOP senator with knowledge of the conversations, was that Republicans were on the front lines of the trial and for them to be left without all available information was "malpractice."

McConnell, a spokesman said, "did not have any advance notice" of the manuscript.

Bolton has flirted with testifying in the Senate impeachment trial for several weeks now after Democrats say his lawyer threatened to file suit if he were subpoenaed to testify during the House impeachment inquiry.

Earlier this month, Bolton said he was willing to testify in the Senate if he was subpoenaed — a development that looked unlikely when the Senate recessed on Saturday but is now up in the air.

Trump's defense continues

On the Senate floor, Sekulow argued that Democrats conducted "a pattern and practice of attempts over a three year period to not only interfere with the President's ability to govern," which he called "unsuccessful."

Sekulow said the team, including former independent counsel Kenneth Starr, who led the investigation leading to President Bill Clinton's impeachment, would begin by laying out an overview of "historical and constitutional issues with impeachment proceedings."

Starr warned senators they were in what "can aptly be described as the age of impeachment," as he argued that Trump's impeachment was setting a dangerous standard, failed to charge the President with a crime and moved forward despite lacking any Republican support.

"Like war, impeachment is hell, or, at least, presidential impeachment is hell," Starr said. "Those of us who lived through the Clinton impeachment understand that in a deep and personal way."

Pam Bondi, the former Florida attorney general and a member of the President's defense team, used her 30-minute presentation on the Senate floor to outline Hunter Biden's involvement on the board of Burisma, a Ukrainian natural gas company, the first direct reference to the Bidens during the defense team's presentations.

House managers, she said, "repeatedly referenced" Biden and Burisma more than "400 times" during their presentations last week.

"We would prefer not to be talking about this," Bondi claimed, "But the House managers have placed this squarely at issue, so we must address it."

Citing multiple news reports and testimony from witnesses, Bondi cast the company as corrupt and Biden's involvement as a conflict of interest. She questioned his qualifications to serve on the board, an opportunity she called "nepotistic at best, nefarious at worst."

Trump's attempts to pressure Ukraine to investigate Hunter Biden and Joe Biden, his potential political rival, are at the center of the President's impeachment trial.

Trump has repeatedly made unfounded and false claims to allege that the Bidens acted improperly in Ukraine. There is no evidence of wrongdoing by either Joe or Hunter Biden.

Deputy White House counsel Patrick Philbin argued that the impeachment article regarding obstruction of Congress "will forever damage the separation of powers" and considering it an impeachable offense would "itself be an abuse of power by Congress."

And in her first remarks during the trial, Trump lawyer Jane Raskin offered a robust defense of Rudy Giuliani, saying that House managers did not provide "any direct evidence" incriminating the former New York City mayor and insisting he was not on a "political errand" for Trump, but rather, defending his client.

"Yes, Mayor Giuliani was President Trump's personal attorney, but he was not on a political errand. As he stated repeatedly and publicly, he was doing what good defense attorneys do, he was following a lead from a well-known private investigator, and he was gathering evidence regarding Ukrainian election interference to defend his client against the false allegations being investigated by special counsel Mueller," Raskin said.

She described Giuliani as a "minor player -- that shiny object designed to distract you."

The mood in the Senate chamber was somber as the defense team presented their argument, with Republicans and some Democrats taking copious notes. Senators were present and, largely, attentive, with no absences and no signs of any fidget spinners.

On Saturday, the defense team spent two hours walking through an overview of its argument, seeking to poke holes in the House's case — including that the House didn't have any direct evidence linking the President to a quid pro quo.

What happens next

After the defense team concludes its arguments, there will be 16 hours for senators to ask questions of both sides. The Senate's trial rules then call for a debate and vote on whether the Senate generally should seek more witnesses and documents.

If that vote fails, the trial is likely to conclude with an acquittal vote this week. But if it succeeds, the trial will enter into an unpredictable phase where both senators and the legal teams could propose witnesses, which the Senate could vote on.

In addition to the debate over Bolton's testimony, Schiff argued that the Senate must also seek his notes — as House Democrats were stonewalled from receiving documents from the Trump administration during the impeachment inquiry, even from witnesses who testified before the House.

"We learned John Bolton took detailed notes and presumably these are contemporaneous. These notes took place while the events were happening, while they were fresh in his mind. Those in many respects are more important than the manuscript," Schiff said. "We ought to not only have John Bolton testify but we ought to see what he wrote down in his notes at the time."