"At this point, the president's conduct in office should not surprise us. But truly devastating has been our tolerance of that conduct," Flake wrote in an op-ed.
Trump has no shortage of opponents, but here are some Republicans who have made their opposition known.
"What I would like is for the Republican Party to nominate someone else."
Barkan, who has ALS, once confronted Senator Jeff Flake on a plane about proposed tax cuts.
Another four years of Trump could doom the Republican Party for good, says former Arizona Senator Jeff Flake.
"Kyrsten Sinema won because she ran a good race and had a message that resonated with Arizona's voters," Flake said.
McConnell believed the measure to protect Mueller, a bipartisan bill that would curb Trump's ability to interfere with Mueller's investigation or to fire the special counsel, is "blatantly unconstitutional."
Flake's vote against advancing a judicial nominee came after GOP Senator Mike Lee blocked a move by Flake to bring a Mueller protection bill to the Senate floor for a vote, something GOP Senate Majority Leader Mitch McConnell has refused to do.
"At the end of the day, I am convinced that Mr. Mueller will be allowed to do his job," the senator said.
Republican senators have joined Democrats' calls to postpone Brett Kavanaugh's Supreme Court confirmation vote until the woman who has accused him of sexual assault can be heard by senators on the Judiciary Committee.
"Sorry, [Donald Trump]. I made no endorsement in this race," Jeff Flake responded on Twitter.
"And only a passing familiarity with how the Constitution works."
Former GOP communications director Tara Setmayer said Joe Arpaio's Senate run could be "Roy Moore part two."
President Donald Trump could see a packed stage of Republican challengers again in 2020—only as an incumbent this time.
Jeff Flake suggested President Donald Trump's agenda posed a threat to the future of the Republican Party.
The Republican senator said he "couldn't sleep at night having to embrace the president."
Two Democrats added Jeff Flake to the not-so-exclusive club after he criticized Donald Trump.
The chorus of GOP Senators speaking out against Trump exposes deep divides ahead of the 2018 midterms.
Republican candidate Ward has the backing of Steve Bannon and Robert Mercer.
The Arizona lawmaker said there was no place for a Republican like him in the GOP.
Donald Trump vowed to provoke a government shutdown unless Congress funds his border wall, but it may be averted.