(CNN) The revelations contained in the draft manuscript of former national security adviser John Bolton have upended the impeachment trial of President Donald Trump, but they received nary a mention inside the Senate chamber on Monday -- until the very end of the day.

The President's legal team focused its presentation on attacking the House impeachment managers' case and going after former Vice President Joe Biden and his son Hunter Biden. Then, in the final presentation, Trump lawyer and Harvard Law professor emeritus Alan Dershowitz addressed the elephant in the Senate chamber.

"Nothing in the Bolton revelations -- even if true -- would rise to the level of an abuse of power or an impeachable offense," Dershowitz said.

"Even if the President, any president, were to demand a quid pro quo as a condition to sending aid to a foreign country, obviously a highly disputed matter in this case, that would not by itself constitute an abuse of power," he argued, adding, "Quid pro quo alone is not a basis for abuse of power, it's part of the way foreign policy has been operated by presidents since the beginning of time."

Dershowitz spoke for just over an hour Monday evening in the final presentation of the day, arguing that abuse of power was a "political weapon," not an impeachable offense.

The White House's defense will resume at 1 p.m. ET Tuesday for its final allotted day for opening arguments.

While Bolton was barely mentioned during the proceedings Monday, the allegations that Trump had told him US security aid to Ukraine was conditioned on that country investigating the President's political rivals dominated the conversation outside the chamber among the senators who will decide whether witnesses will testify in the impeachment trial.

Two key moderate Republicans said the Bolton news strengthened the case for having witnesses at 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 and stayed firm in opposition to witnesses.

Democrats ramped up their push for witnesses in light of the Bolton news, and some Republican senators discussed whether there should be a trade if witnesses are called.

It's a conversation that has shifted far from where the President's lawyers and GOP senators thought they would be going this week, after Republicans had appeared confident when leaving Washington on Saturday 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.

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

"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.

Instead, the defense team focused its energy on the Bidens, using two presentations to attack Joe Biden and his son for Hunter Biden's taking a job on the board of Burisma, a Ukrainian energy company.

Both Romney and Sen. Susan Collins of Maine said the Bolton news had strengthened the case for witnesses, and that they were likely to support it. Other key GOP moderates remained on the fence: Sen. Lisa Murkowski of Alaska said she was "still curious" about Bolton's testimony and Sen. Lamar Alexander of Tennessee said he'd make a decision when the opportunity to vote on witnesses comes.

Republicans mixed on Bolton allegation

Underscoring how unsettled the Bolton revelations had left some in the Senate Republican conference, Sen. Pat Toomey proposed the idea of a "one-for-one" witness trade with Democrats during the party's closed-door lunch. The idea has been pinging around among Republicans for weeks, but the Pennsylvania Republican's decision to bring it up during the GOP lunch, which was first reported by The Washington Post , signaled the fluidity of the moment as senators attempted to grapple with the developments.

Senate GOP aides cautioned that the idea, at least at this point, is premature and Republican leaders urged the conference to hear out the remainder of the White House presentation before considering any deals or agreements. Senate Majority Leader Mitch McConnell, for his part, told his conference to "take a deep breath," according to senators in the room, making clear that the trial rules allow for a witness vote later in the process, and decisions should be held off until then.

It remains the Kentucky Republican's strong preference, sources say, to defeat the witness vote altogether and he's continued to warn his members of the fallout of moving toward subpoenas for witnesses and documents, ranging from an elongated trial to creating new precedents on impeachment and executive privilege they may come to regret.

At this point, GOP sources say the four Republican votes necessary to move forward on witnesses haven't come to the forefront, though three are considered a near-certainty right now: Collins, Romney and Murkowski. The fourth, however, "hasn't shown his or herself, so everyone's just trying to feel their way through this," one Republican senator told CNN.

In the hours after The New York Times broke the Bolton story , Republican senators began reaching out to the White House to control the damage. Both Sen. Lindsey Graham of South Carolina and McConnell were in communication and there was an expressed view that the GOP-led 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."

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.

The California Democrat, 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's defense continues

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

On the Senate floor, the President's legal team focused elsewhere. 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."

Early in the day, former independent counsel Kenneth Starr , who led the investigation leading to President Bill Clinton's impeachment, 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, 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.

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 the Bidens 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.

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 few absences and no signs of any fidget spinners.

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."

This story has been updated with additional developments Monday.