The independent congressman from Michigan was elected as a Republican, but formally withdrew from the political party in July.
GOP Nebraska state Senator John McCollister fired back at Trump after the president called the impeachment inquiry "the greatest con job in the history of American politics."
Senator Chuck Schumer says he expects "some" of his Republican colleagues to say "that's fair," when they read his letter.
"The conduct of [Mitch McConnell] in this impeachment trial is corrupt and partisan beyond the pale," Richard W. Painter tweeted.
"The president has a very unorthodox style of messaging. It's not the way that I talk or I would tweet," Republican Congressman Mike Johnson said.
"How impartial can it be when McConnell says, quote, he is taking his 'cues' from the White House," Chris Wallace asked Pam Bondi.
"I think it is the responsibility of the American people to hold these senators accountable," the congresswoman from Minnesota said.
David Richter, a GOP candidate running for Congress in New Jersey's 2nd District, called Van Drew an "absolute weasel."
"You can't hide from history," S.E. Cupp warned.
"They do not agree with" Trump and "think that the course that he is on is wrong for America," Rep. Denny Heck said
A representative for Republicans for the Rule of Law told Newsweek that the group targeted representatives who have shown a willingness to "assert their independence" of the president.
"They have decided to put party above the rule of law, party above what is right," Rep. Mike Quigley said.
A California GOP challenger in Maxine Waters' 43rd District was arrested in San Francisco Saturday after violating a restraining order placed against him by a fellow GOP candidate running against House Speaker Nancy Pelosi.
"Republicans were left floundering in their defense of a president who kept admitting to what he was accused of," S.E. Cupp said.
Pence told Fox News that he was "not only confident" that he'd be on the 2020 ticket, but that he was also "confident that we're going to win in 2020."
"I think a full blown defense that says 'this is a hoax' and 'there's absolutely nothing to it,' will be difficult for them to sustain," Senator Chris Coons said.
"This is the most desperate defense," Joe Scarborough said.
President Donald Trump's Republican allies have propagated a baseless conspiracy theory that Ukraine meddled in the 2016 presidential elections.
"We have a president of the United States ... who literally just makes s--- up," Joe Walsh said.
"It's lack of character among people on these committees. Just a lack of character," MSNBC host Joe Scarborough said.
"If you find yourself lying about a war hero, you should probably question your choices in life," Representative Sean Maloney said.
During the impeachment hearings, President Trump has attacked Maria Yovanovitch on Twitter, as well as other officials who have testified, and his party worries it may backfire.
"He's been racist, he's been a jerk," Ana Navarro said, "he's attacked people like me, he's attacked Donna Brazile, he's attacked Don Lemon."
"Well wait a minute, no sir," Chris Wallace said. "They're career foreign service officers and these are people who worked in the Trump administration."
"That's at least one promise that they fulfilled," quipped "Morning Joe" host Joe Scarborough on his MSNBC show.
"We have a situation where we see the national security of the United States ... compromised over the president's political interests," Steve Schmidt said.
Former Obama and Trump ethics chief Walter Shaub denounced the language Republicans have been using to discuss the alleged whistleblower. "The party won't be satisfied until somebody gets killed," he wrote.
Republicans "are going to try and discredit the witnesses. But as Sam Stein said, they are unimpeachable witnesses," Susan Del Percio said
"I think it's really regrettable that now the president and his supporters and others are advocating his [or her] unmasking," the former Director of National Intelligence James Clapper told CNN.