Most impeachment trial watchers on Monday, however, were following a major development that occurred outside of the Senate chamber.
A report in The New York Times about the alleged conversation between Trump and former National Security Adviser John Bolton — contained in a draft of the former national security adviser's book manuscript — shifted the focus to whether senators would vote to call witnesses, including Bolton, as part of the proceedings.
The report was based on Bolton's forthcoming book, a draft of which had been sent to the White House as part of a standard review process.
As speculation grew about whether Republicans would support calling Bolton, Dershowitz referenced the former top Trump official and dismissed the portions of his yet-to-be-published book that have grabbed the headlines.
"Nothing in the Bolton revelations, even if true, would rise to the level of an abuse of power or an impeachable offense," he said.
Two Republican senators, Mitt Romney of Utah and Susan Collins of Maine, said Monday that the revelations about Bolton's book have made it more likely that senators will want to call Bolton to testify.
"I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton," Romney said, noting that he would make his final decision on the issue after hearing from Trump's defense.
Collins tweeted that the reports about Bolton's book "strengthen the case for witnesses and have prompted a number of conversations among my colleagues."
Two others who are seen as more open to calling witnesses — Sens. Lisa Murkowski of Alaska and Lamar Alexander of Tennessee — were tight-lipped.
Alexander said senators will decide "to call witnesses or other documents if we needed them after we hear all the arguments." Murkowski said in a tweet that while she was "curious" about what Bolton had to say, there is "an appropriate time for us to evaluate whether we need additional information."
Jay Sekulow, Trump's personal attorney, argued that the president "was at all times" acting under his constitutional and legal authority by withholding aid to Ukraine, saying that Trump's impeachment is about "deep policy concerns."
Ken Starr, the former independent counsel who led the impeachment efforts against President Bill Clinton in 1998 and is now part of Trump's defense team, began his arguments by reciting a history of impeachment and declared, "Like war, impeachment is hell." He said, "It divides the country like nothing else. Those of us who lived through the Clinton impeachment understand that in a deep and personal way. "
Starr went on to accuse the House of abusing its power by approving article 2, which charges that Trump obstructed Congress.
"Here, we have, tragically for the country and, I believe, tragically for the House of Representatives, in article 2 of these impeachment articles, a runaway House," Starr said. "It has run away not only from its long-standing procedures — it has run away from the Constitution's demand of fundamental fairness."
Another of Trump's attorneys, Jane Raskin, spoke to defend Rudy Giuliani, Trump lawyer who was painted by the House managers as leading a rogue foreign policy operation by pursuing information against former Vice President Joe Biden and his son, as well as pressuring to remove former U.S. Ambassador to Ukraine Marie Yovanovitch.
Giuliani, said Raskin, was the House's "colorful distraction," and, "a shiny object designed to distract you."
She said the "House managers may not like his style, you may not like his style. But one might argue, that he is everything that Clarence Darrow said a defense lawyer must be, outrageous, irreverent, blasphemous, a rogue, a renegade."
Later in the afternoon, Trump attorney Pam Bondi, a former Florida attorney general, rose to attack former Vice President Biden and his son Hunter's dealings in Ukraine. The younger Biden was hired to sit on the board of a Ukrainian gas company Burisma at the same time his father was the Obama administration's point man in fighting corruption there. In the infamous July 25th phone call, Trump asked Ukraine's president to "do us a favor, though" and investigate the Bidens. The first article of impeachment charges Trump with abusing the power of his office by asking for the investigation in return for releasing nearly $400 million in military aid to Ukraine. Trump denies any such link was made, though Bolton's book appears to undercut that assertion.
Bondi said the younger Biden was paid $83,000 a month to sit on the Burisma board, and played a news clip in which Hunter was asked if would have been hired by the firm if his last name wasn't Biden. "I don't know, I don't know, he responded. Probably not."
The Push for Witnesses
Over three days of arguments last week, House impeachment managers outlined their case that Trump enlisted a foreign power to help him cheat in the 2020 election and then has tried to cover it up. They've said that the president's conduct is the "worst nightmare" of the nation's Framers.
Trump's legal team is facing a mostly sympathetic group of jurors in the Republican-controlled Senate. An acquittal on the two articles of impeachment is all but guaranteed, since the Democratic caucus would need 20 GOP members' support to have the two-thirds majority needed to convict and remove the president.
More up in the air, however, is whether Democrats will be able to recruit enough Republicans to call witnesses for the trial. That vote has a lower threshold than the one required to remove the president from office. At least four GOP senators would have to cross party lines to call witnesses or subpoena documents.
In light of the Times' report on Sunday, Democrats were quick to reiterate that Bolton should testify.
Senate Minority Leader Chuck Schumer, D-N.Y., said Monday that the revelations should remove the GOP resistance against witnesses. He said details in the book provide "further evidence that a large number of people were in the loop" on Trump's alleged pressure campaign on Ukraine.
Schumer said there seems to be a "giant cover-up" within the White House and added that if Republicans don't vote to call witnesses, including Bolton, "they are going to be part of the cover-up too."
Sen. Angus King, I-Maine, told NPR on Monday that up to 10 Republican senators could vote for witnesses following the Times' report about the Bolton manuscript.
"I'm already hearing a number of Republicans who are moving toward voting to at least hear from John Bolton, if not other witnesses," he said. When asked if four Republicans would vote for witnesses, he said: "I think there'll be more. My bold prediction will be five or 10."
Rep. Adam Schiff, D-Calif., the lead impeachment manager, linked testimony from Bolton to the very notion of a fair trial. "To say we will blind ourselves from a witness who has so clearly relevant testimony to one of the central most serious allegations against the president — I don't see how we can have a fair trial without testimony like that," he told reporters on Monday before the White House lawyers continued their arguments.
House Democrats had asked Bolton to testify during the impeachment inquiry but did not issue him a subpoena. The White House had ordered senior officials to not testify.
Any vote on witnesses and new evidence would come after the defense team rests (it has up to 24 hours over three days) and after senators have a chance to ask questions in writing of both House prosecutors and Trump's legal team. The questions will be read out loud by Chief Justice John Roberts, who's presiding over the trial.
Bolton's Manuscript
NPR has not seen a draft of the manuscript and cannot vouch for the accuracy of the report on Trump's comments to the former national security adviser. A spokesman for the National Security Council said no one at the White House other than people at the NSC had reviewed Bolton's book.
"Ambassador Bolton's manuscript was submitted to the NSC for pre-publication review and has been under initial review by the NSC," NSC spokesman John Ullyot said in a statement. "No White House personnel outside NSC have reviewed the manuscript."
Trump denied Bolton's account in a series of tweets.
"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," he said. "If John Bolton said this," Trump continued, "it was only to sell a book."