Arizona senator Jeff Flake has become the latest member of the Republican Party to come out against US President Donald Trump.
Senator Flake said President Trump's "impulse to threaten and scapegoat" could turn the US into a "backward people."
Announcing that he would not run for re-election next year, he said American politics had become hardened to "reckless, outrageous and undignified behaviour from the White House."
In a dramatic speech on the Senate floor, he lamented what he called the "flagrant disregard of truth and decency" in politics, saying: "Heaven help us."
Debasing the nation
His speech comes after Republican Senator Bob Corker, the chairman of the Senate foreign relations committee, declared he would not seek re-election and began a bitter war of words with President Trump.
Senator Corker called the US President a liar who has damaged the country's standing in the world.
In an interview on Good Morning America, he again warned that the White House had become an “adult day care centre,” and said the president’s conduct was “debasing” the US.
White House response
The White House responded to Senator Flake’s speech by saying it was "probably a good move" that he was not running for re-election.
Mr Flake admitted his decision to leave America's upper chamber was motivated by his being out of step with his party.
"There may not be a place for a Republican like me in the current Republican climate or the current Republican Party," he said.
He went on to say that there were times when one must put principles and one’s career ahead of party politics – adding that he could no longer remain silent.
He said the efficacy of the US leadership around the globe was coming into question as the country gave up on its principles in favour of anger and resentment.
The "instinct to scapegoat and belittle threatens to turn us into a backward-looking people", he said.
Senator Flake’s decision not to seek re-election came following a host of similar decisions from Republicans in swing states.
Some of those who have announced plans to retire include David Trott of Michigan, Charlie Dent of Pennsylvania, David Reichert of Washington and Ileana Ros-Lehtinen of Florida.