Former House Republican Conference policy director Evan McMullin criticized Alan Dershowitz's defense during Monday's hearing.
"It's pretty fair to say that John Bolton has a relevant testimony to provide," the Republican senator told reporters.
The topic was among the top Twitter trends as users posted their reasons for leaving the party, with many placing the blame on President Donald Trump.
Democrats said Trump attorneys' opening statements had the unintended result of making the case for calling more witnesses.
"The evidence is ample and uncontradicted" for removing the president from office, Andrew Napolitano argued.
Twitter users voiced frustration at the tactics employed by Republican senators, seeking to block new witnesses.
Georgia Democratic Representative Lucy McBath ran for office after her son was shot and killed by a man who claimed he was playing his music too loudly.
"I think we ought to go through the right process," the lawmaker from Florida said.
The Eastern Arizona Courier compared the senator's behavior to "name-calling like a third-grader on the playground."
"I think they [the Senate] should hear the testimony that they can get," Congressman Francis Rooney said.
"That directly contradicts what you said as a Republican House impeachment manager in 1999 during the Clinton impeachment trial," Chris Wallace noted.
"It's also pretty damn cowardly. Imagine being this afraid," S.E. Cupp said.
Chip Roy instructed House Speaker Pelosi how to walk from the House of Representatives to the Senate in a video expressing GOP frustration at her weeks-long decision to hold onto the articles of impeachment.
Democrats are keen to hear from key administration official witnesses, but Republicans led by Senator Mitch McConnell are fighting the proposal.
"I find that absolutely insane. I think that's unacceptable," Lee said.
Democratic Congresswoman Maxine Waters tweeted Monday that Republicans who "shield and protect" President Donald Trump are "responsible for dragging us to war."
GOP Senator Roy Blunt claimed that President Donald Trump's Senate impeachment trial will be over before his State of the Union address in early February.
Former Ted Cruz aide Rick Tyler told MSNBC that the party had done a "really bad job" of recruiting minority members.
"I don't like the way that he tweets, some of the things that he says," GOP Senator James Lankford said. "His word choices at times are not my word choices."
"But I think anyone voting on the facts, anyone voting on the law, this is a very easy vote," the senator from Texas argued.
"Where's the Republican leadership in Congress to say 'no'?" Senator Ben Cardin asked in an interview with Fox News Sunday.
"I think Senator McConnell is entitled to his opinion and his approach, so is Senator Murkowski," Kennedy told CNN.
Steve Scalise accused Speaker Pelosi of trying to negotiate her own "quid pro quo" in the Senate. He also contradicted Mitch McConnell's own comments, claiming the Senate "will hold a fair trial."
Alaska Republican Senator Lisa Murkowski said she was "disturbed" after hearing Senate Majority Leader Mitch McConnell and other top GOP senators were coordinating the impending impeachment trial with the White House.
The conservative attorney's remarks came in response to House Minority Leader Kevin McCarthy's assertion that the FBI investigation of Trump's campaign was a "modern-day Watergate."
Claire McCaskill noted that during her "first year in the Senate," she had voted on more than 300 legislative amendments. "This year, there's fewer than 30," she said.
"You truly represent what news media should be about," the congressman told Fox host Maria Bartiromo.
"It doesn't have to be an either or," Marc Short replied. "it could be both."
"That's really dangerous," Republican Representative Adam Kinzinger warned.
The independent congressman from Michigan was elected as a Republican, but formally withdrew from the political party in July.