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

A pool of reporters inside the Senate chamber observed that nearly every seat was filled when the House managers arguments began at noon and that almost every senator wore a mask. One notable absence was Sen. John Hawley, R-Mo.

Some senators, such as Sens. Ben Sasse, R-Neb., Lisa Murkowski, R-Alaska, and and Pat Toomey, R-Pa., intently took notes as the House impeachment managers made this assertions. Though reporters noted that the vast majority of senators appeared to be at least partially engaged with the content arguments from Rep. Raskin, D-Md., and Rep. Joe Neguse, D-Colo. 

During the prayer and pledge, as Chaplain Barry Black said, “during this impeachment trial, give our lawmakers the gift of discernment so that they will know truth from falsehood,” several seats on both sides of the aisle were empty. Republicans who were there bowed their heads while standing over their desks, and Democrats, like Sens. Cory Booker, D-N.J. and Maize Hirono, D-Hawaii, were gathered in the back of the chamber.

Sen. Tammy Baldwin, D-Wis., sat on the Republican side talking to Sens. Ron Johnson, R-Wis., and Toomey before going to her seat on the other side of the aisle. After the pledge, once Rep. Jamie Raskin, D-Md., stepped up to the microphone, Democrats and Republicans filed in from the cloakrooms -- some of them still chewing underneath their masks.

Some Republicans came in late, like Sens. Ted Cruz, R-Texas, Roy Blunt, R-Mo., Susan Collins, R-Maine, and James Risch, R-Idaho. Hawley, when he came in alone, watched from the gallery above. He was seen flipping through manila folders and -- at one point -- put his feet up on the desk. He rarely looked at the arguments on the Senate floor.

Senate Minority Leader Mitch McConnell sat with his hands folded in front of him.

When House managers used video exhibits, reporters noted that many GOP senators took notice. During some videos, they described every GOP senator turning and watching the exhibit or reading the exhibits for an extended period of time. 

-ABC News' Katherine Faulders

The Senate is taking a 15-minute break following the start of opening arguments from the House impeachment managers.

Rep. Eric Swalwell, D-Calif., laid out the framework for how Trump, in the days after the election continued to falsely tell his supporters the election was "rigged" and "stolen," using a series of Trump's own tweets and remarks to make his point.

"Just like to build a fire, it doesn't just start with the flames, Donald Trump for months and months assembled the tend, the kindling, threw on logs for fuel to have his supporters believe that the only way their victory would be lost was if it was stolen. That way, President Trump was ready if he lost the election to light the match," Swalwell said.

Swalwell went on to remind how some election officials across the country were berated in the weeks following the election and played video of protesters surrounding the secretary of state's house in Michigan.

"There is plenty of evidence to show words have consequences -- and if he wanted to stop it, he could stop it," Swalwell said. "President Trump was never shy about using his platforms to try and stop something. He could have very easily told his supporters, stop threatening officials, stop GOP going to their homes, stop it with the threats. But each time he didn't."

Swalwell said Trump did the "polar opposite" of what anyone else in that chamber would have done had they lost an election. He started running ads falsely claiming election fraud and spent $50 million from his legal defense fund to amplify his message, Swalwell said.

"These ads were designed to run all the way up to Jan. 5, and then, they stopped. This was purposeful and deliberate planning to target his base to rally around that day. And it wasn't just his ads. He continued to use his own platform. He told his supporters who truly believed their victory had been stolen and who were ready to fight when, where and how to stop what he believed was a steal," Swalwell said.

Rep. Joaquin Castro, D-Texas, one of the House impeachment managers, asserted that Trump's baseless claims of election fraud well before and after the election provoked his base and led to the riots on Jan 6.

"In tweet after tweet, he made sweeping allegations about election fraud that couldn't possibly be true. But that was the point. He didn't care if the claims were true," Castro said. "He wanted to make sure that his supporters were angry, like the election was being ripped away from them."

Castro argued that the president's "big lie" about election fraud fanned the flames of anger in his base, leading to the riots.

"There's a saying that a lie can travel halfway around the world before the truth has a chance to put on its shoes. And that was before the internet," Castro said. "The point of that saying is that a lie can do incredible damage and destruction, and that's especially true when that lie is told by the most powerful person on earth, our commander-in-chief, the president of the United States."