Republican Senator Jeff Flake has blasted President Donald Trump for his "reckless, outrageous and undignified behaviour".
Arizona's Flake said he would not seek re-election next year, because he did not want to seem "complicit" with Trump's policies.
In his resignation speech, Flake said: "We must never meekly accept the daily sundering of our country - the personal attacks, the threats against principles, freedoms, and institutions, the flagrant disregard for truth or decency, the reckless provocations, most often for the pettiest and most personal reasons, reasons having nothing whatsoever to do with the fortunes of the people that we have all been elected to serve...
"We were not made great as a country by indulging or even exalting our worst impulses, turning against ourselves, glorying in the things which divide us, and calling fake things true and true things fake."
Flake's public criticism of Trump came soon after Tennessee Senator Bob Corker labelled the President "utterly untruthful".
Trump has received a lot of criticism from politicians on both sides recently over his phonecalls to families of fallen soldiers.
He particularly offended many when he disputed grieving military widow Myeshia Johnson's description of their phonecall as upsetting, essentially calling her a liar.
Corner said: "He is purposely breaking down relationships we have around the world that had been useful to our nation.
"I think the debasement of our nation is what he'll be remembered most for."
Corker added the President has "great difficulty with truth".