In this screenshot taken from a Senate Television webcast, Legal Counsel for President Donald Trump, Alan Dershowitz speaks during impeachment proceedings against U.S. President Donald Trump in the Senate at the U.S. Capitol on January 27, 2020 in Washington, DC. Senate Television | Getty Images

President Donald Trump's lawyers laid out a multi-pronged defense in the Senate impeachment trial Monday, pushing back on House Democrats' case for Trump's conviction and removal from office. The defense team sought to undermine the arguments presented last week by the House impeachment managers, while making the historical case for Trump's acquittal and rationalizing his dealings with Ukraine. They also presented expansive arguments defending Trump's lawyer, Rudy Giuliani, and raising suspicions about the conduct of Hunter Biden, the son of 2020 Democratic presidential candidate Joe Biden. But as of 8 p.m. on Monday evening, Trump's lawyers had not addressed former national security advisor John Bolton's reported claim that Trump withheld nearly $400 million in military aid to Ukraine in order to secure investigation into his political opponents. Trump denied Bolton's claim, reportedly made in a manuscript of his upcoming memoir. The defense team has 24 cumulative hours to present its opening statements. The lawyers kicked off their arguments in an abbreviated session Saturday morning, accusing Democrats of pursuing Trump's impeachment simply to remove him from the ballot in the 2020 presidential election. The House voted last month to impeach Trump on articles of abuse of power and obstruction of Congress, both related to his dealings with Ukraine. It remains unlikely that two thirds of the Senate will vote to remove Trump, as the Constitution requires. Here are the day's biggest moments from the Senate trial:

GOP senators say report on Bolton's book boosts need for witnesses in trial...

The New York Times reported Sunday that Bolton's claims in an upcoming book that Trump said he wanted a military aid package to Ukraine withheld until Kyiv agreed to announce investigations into his Democratic political opponents, including the Bidens. Trump denied the reported allegations on Twitter early Monday morning. CNBC has not seen a copy of Bolton's manuscript. Democrats have held Bolton up as a key witness who must be heard in the trial. And the handful of Republicans seen as being moderate enough to side with the Democrats on witnesses appeared to be taking Bolton's reported claims seriously.

U.S. Senator Mitt Romney (R-UT) speaks to the media while walking towards the Senate Chamber for the Senate Impeachment Trial in Washington. Michael Brochstein | Getty Images

Republican Sen. Mitt Romney of Utah told reporters Monday morning that "I think it's increasingly likely that other Republicans will join those of us who think we should hear from John Bolton." Sen. Susan Collins, R-Maine, released a statement saying the reports on Bolton's book "strengthen the case for witnesses." They "have prompted a number of conversations with my colleagues," Collins said. Collins TWEET Republican Sen. Lisa Murkowski of Alaska tweeted: "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." Murkowski TWEET Senate Minority Leader Chuck Schumer, D-N.Y., said Monday morning that the new reports erase even a "shred of logic left to not hear witnesses and review the documents."

...But Trump's defense sidesteps Bolton's claims

The president's lawyers hammered the evidence gathered during the House impeachment inquiry from numerous current and former administration officials. They argued that nearly all of the testimony in those proceedings came from second- or third-hand sources who did not speak directly to Trump about his intentions toward Ukraine. But they didn't mention Bolton's newly reported allegations, which loomed over the impeachment proceedings Monday. "Not a single witness testified the president himself said that there was any connection between any investigation and security assistance, a presidential meeting, or anything else," Trump lawyer Jay Sekulow said.

In this screengrab taken from a Senate Television webcast, Legal Counsel for President Donald Trump Jay Sekulow speaks during impeachment proceedings against U.S. President Donald Trump in the Senate at the U.S. Capitol on January 25, 2020 in Washington, DC. Senate Television | Getty Images

It's true that none of the witnesses in House proceedings testified that Trump himself explicitly tied the Ukraine aid to the political probes. But the White House had pressured some potential witnesses not to cooperate with the impeachment inquiry, and others — including Bolton and acting White House chief of staff Mick Mulvaney — have refused to comply.

Rudy Giuliani gets a robust defense

Trump's defense team offered a lengthy, rigorous defense of the president's personal lawyer, whom they argue "is the House managers' colorful distraction." "House managers would have you believe that Mr. Giuliani was at the center of this controversy," Trump lawyer Jane Raskin told senators. "If Rudy Giuliani is everything they say he is, don't you think they would have subpoenaed and pursued his testimony?" House Democrats subpoenaed documents from Giuliani in September during their investigation. But a lawyer for Giuliani told the House in October that Giuliani would not comply with the impeachment inquiry, calling it "unconstitutional, baseless, and illegitimate." Raskin, referring to Giuliani as a "minor player," said he wasn't on a "political errand" while in talks with Ukraine on digging up potentially damaging information about Joe Biden. Rather, she said, he was being a good defense lawyer for the president. "He was gathering evidence regarding Ukrainian election interference to defend his client against the false allegations being investigated by Special Counsel Robert Mueller," Raskin said. Giuliani reacted positively to Raskin's defense of him, saying in a tweet following her arguments that he "did not dig up dirt on Joe Biden." Giuliani tweet Federal prosecutors in New York are investigating Giuliani to determine whether he broke laws regulating foreign lobbying following the ouster of former U.S. ambassador to Ukraine Marie Yovanovitch.

Pam Bondi argues the Bidens merit further investigation

Former Florida Attorney General Pam Bondi took the floor Monday afternoon to argue one of the most controversial portions of Trump's defense, and one that several Republican senators still do not consider credible: The claim that former Vice President Joe Biden and his son Hunter Biden's activities in Ukraine were suspicious and worthy of further investigation.

In this screengrab taken from a Senate Television webcast, Legal Counsel for President Donald Trump, former Florida Attorney General Pam Bondi speaks during impeachment proceedings against U.S. President Donald Trump in the Senate at the U.S. Capitol on January 27, 2020 in Washington, DC. Senate Television | Getty Images

For more than half an hour, Bondi laid out the discredited theory that in 2014, Joe Biden had pressed for the removal of a corrupt Ukrainian prosecutor, not because the prosecutor was corrupt, but because that prosecutor had run a corruption probe of Burisma, a Ukrainian gas company who employed Hunter Biden as a board member. But the timeline of this theory doesn't line up -- the investigation into Burisma was closed before Hunter Biden joined the board. Nonetheless, last summer Trump asked Ukrainian president Zelenskiy to launch a new probe of the Bidens, a request at the heart of the current impeachment case against Trump. Bondi argued Monday that Democrats "might say, without evidence, that everything we just have said has been debunked, that the evidence points entirely and equivocally in the other direction." Still, Bondi said, "all we're saying is that there was a basis to talk about [the Bidens], to raise this issue" with Zelenskiy. "And that is enough." A spokesman for Joe Biden's 2020 presidential campaign dismissed Bondi's argument. "Here on Planet Earth, the conspiracy theory that Bondi repeated has been conclusively refuted," said rapid response director Andrew Bates.

Ken Starr, who led Clinton probe, decries "age of impeachment"

Ken Starr, the former head of the Independent Counsel's Office which investigated then-President Bill Clinton's extramarital affair in the 1990's, returned to the Senate floor Monday to argue a point that few people ever expected to hear coming from Starr: Presidential impeachment has been weaponized, Starr told senators, likening the process to a "domestic war" that "divides the country like nothing else."

In this screengrab taken from a Senate Television webcast, Legal Counsel for President Donald Trump Ken Starr speaks during impeachment proceedings against U.S. President Donald Trump in the Senate at the U.S. Capitol on January 27, 2020 in Washington, DC. Senate Television | Getty Images