A US Republican senator says he will not seek re-election, delivering a fierce attack on President Donald Trump.
Arizona Senator Jeff Flake said "reckless, outrageous and undignified behaviour" at the top of the US government was dangerous to democracy.
Mr Trump has previously called Mr Flake "toxic".
The US president is already embroiled in a row with another Republican Senator, Bob Corker.
Before confirming his decision in a speech to the Senate, Mr Flake told the Arizona Republic newspaper "there may not be a place for a Republican like me in the current Republican climate or the current Republican Party".
Taking to the floor, he said he did not enjoy criticising the president but felt it was "a matter of duty and conscience".
"We must never regard as 'normal' the regular and casual undermining of our democratic norms and ideals," he said.
He lamented the "flagrant disregard for truth or decency, the reckless provocations, most often for the pettiest and most personal reasons".
"I have children and grandchildren to answer to, and so, Mr President, I will not be complicit," he added.
White House press secretary Sarah Sanders said it was "probably a good move" Mr Flake was standing down, suggesting he would not win re-election.
Flake out on his own terms
By Anthony Zurcher, BBC News, Washington
Jeff Flake was facing an uphill battle in his re-election campaign next year. It wasn't because the incumbent senator was matched up against a particularly tough Democrat, but rather because the Republican Party had shifted under his feet.
He found himself at odds with Donald Trump and the populist, nationalist movement that swept the president to power - and facing a right-wing challenger backed by Mr Trump's former adviser and campaign guru, Steve Bannon.
So with poll numbers indicating he very well might be unceremoniously dumped from office, Mr Flake decided to go out on his own terms.
It's become almost a cliché at this point that a Republican Trump critic only finds his voice once his political career is at or near an end.
Mr Flake's situation is different. His career is at an end not by his choice, but by forces outside his control. He's been an outspoken Trump critic, and soon he will be out of a job.
Another prominent party critic of Mr Trump, John McCain, was quick to pay tribute to Mr Flake.
US Republican majority leader Mitch McConnell said he had "witnessed a speech from a very fine man".
Mr Flake has long been a vocal opponent of Mr Trump, refusing to endorse him during the presidential campaign.
Although he largely voted in line with the party, his comparatively moderate views and critiques of the direction of the Republicans under Mr Trump have left him out of kilter with voters who made Mr Trump president.
For his part, Donald Trump has long wanted to oust Mr Flake, even offering to spend millions of his own money to see him unseated in primaries.
In a series of television interviews earlier in the day, Mr Corker accused the president of lying, adding that he had debased the US and weakened its global standing.
Mr Trump fired back on Twitter, calling the Tennessee senator a "lightweight" who "couldn't get re-elected". Mr Corker is also not seeking another term in elections next year.
The feud overshadowed efforts by Mr Trump to build support for his proposed tax cuts. He met Republican senators for a lunch time meeting.