Trump impeachment trial live updates: Biden says charge 'not in dispute' in 1st comments on acquittal

The Senate kicked off former President Donald Trump's second impeachment trial on Tuesday with Democrats using a 13-minute video of the Jan. 6 Capitol riot to help make their case to the Senate, and Trump's defense team debating the constitutionality of the trial of a former president.

Here are three takeaways from the first day of arguments, which ended with the Senate voting 56-44 to proceed with the trial.

Trump's legal team, including Bruce Castor Jr., a former district attorney of Montgomery County, Pennsylvania, made a poor first impression in a 48-minute argument that garnered criticism from senators who found themselves confused and unimpressed.

Senators, many of whom were in the Capitol during the Jan. 6 riots, relived the attacks during a documentary-style video of the Capitol riot, a harrowing presentation that meshed C-SPAN footage of staid House and Senate floor debates with gritty, expletive-laden riot footage and some of the former president's social media posts.

Democrats also cited conservative lawyers who backed the trial, saying that not trying Trump would suggest to future presidents that they would not be held accountable for their actions during their final days in office.

-ABC News' Benjamin Siegel

On Wednesday, the House impeachment managers must file all motions, except those related to witnesses by 9 a.m. and the Trump legal team must to respond to the House managers’ motions by 11 a.m.

The trial will resume at noon on Wednesday with arguments and a vote for any motions made by either side.

After that, opening arguments begin. The House impeachment managers have up to 16 hours over two days, though both sides are limited to eight hours of arguments each day.

-ABC News' Trish Turner

ABC News' Congressional Correspondent Rachel Scott senators on both sides of the aisle looked confused as Trump attorney Bruce Castor spoke for the defense team -- a feeling perhaps shared by the former president who aides told ABC News was unhappy with Castor's performance.

"They were antsy," Scott said of the senators during Castor's remarks, "So much so that several actually got up and left and went to a different gathering place and left the room empty."

She said once David Schoen took over, the senators appeared more attentive to the defense' team's performance.

"But there was a stark contrast to when House Democrats got up to make their arguments when they played that 13-minute long video and there was complete silence in the chamber, so much so that you heard the echoes from the sound from the violent mob that stormed the Capitol that day bouncing off the chamber walls," Scott said.

Scott said that although Democrats may have compelling arguments, the majority of Republicans have already signaled they won't vote to convict Trump. Democrats would need at least 17 Republicans to side with them -- an increasingly difficult feat after Tuesday evening's vote in which 44 Republicans voted the trial was unconstitutional.

"You may have some Republicans who may be moved by the presentation by Democrats they may feel compelled by it. They may be shocked to see what they are seeing before their eyes, what they witnessed watching those events replay for them. But at the end of the day, Democrats are going to need the support of at least 17 Republicans in order to convict Donald Trump and already so many have blasted this process as unconstitutional," Scott said.

Bruce Castor told reporters that he thought Trump's legal defense had a "good day," though other members of the defense team and Republican senators were less enthusiastic.

David Schoen, who followed Castor on the Senate floor, was not so sure of his performance. When asked by reporters how he thought the senators responded to the arguments, he said he "didn't know what they thought." And after learning about criticism from Sen. Bill Cassidy, R-La., Schoen added that he was "sorry he felt that way."

Schoen also responded to a question asking if it meant anything that another Republican the voted with the Democrats on the question about the constitutionality of the proceedings.

"I don't think so. I don't really follow that stuff so carefully, but I was very heartened by some of the comments I got from some of the people. I think they all just vote their conscience, I guess," Schoen said. "They vote how they think best, I guess."

Sens. Susan Collins, R-Maine, and Lisa Murkowski, R-Alaska, who both voted as they did previously on the question, said they were "perplexed" and "stunned" by Castor's presentation. 

 "I thought the first attorney for the president today did not present a case -- which surprised me -- did not make any arguments. The second attorney that represented the president clearly did and did a competent job but I am puzzled by the presentation by the first attorney," Collins said. "So I don't know how they are going to proceed from here on out."

Murkowski said, "I was really stunned at the first attorney who presented for former President Trump. I couldn't figure out where he was going, spent 45 minutes going somewhere, but I don't think he helped with us better understanding where he was coming from on the constitutionality of this."

"And I felt that Mr. Schoen did a, did a better job but I think they sure had a missed opportunity with their first, first attorney there," she added.

-ABC News' Allison Pecorin and Trish Turner