"No longer can we compound attacks on truth with our silent acquiescence. No longer can we turn a blind eye or a deaf ear to these assaults on our institutions," Flake said in his speech
. "An American president who cannot take criticism -- who must constantly deflect and distort and distract -- who must find someone else to blame -- is charting a very dangerous path. And a Congress that fails to act as a check on the President adds to the danger."
Flake also compared Trump's attacks on the news media to the rhetoric of late Soviet dictator Joseph Stalin and highlighted the longstanding consequences for Trump's attacking the truth.
"For without truth, and a principled fidelity to truth and to shared facts, Mr. President, our democracy will not last," Flake said.
He criticized the President for calling the news media the "enemy of the people," calling it "an assault as unprecedented as it is unwarranted."
"It is a testament to the condition of our democracy that our own President uses words infamously spoken by Joseph Stalin to describe his enemies," he said. "It bears noting that so fraught with malice was the phrase 'enemy of the people,' that even Nikita Khrushchev forbade its use, telling the Soviet Communist Party that the phrase had been introduced by Stalin for the purpose of 'annihilating such individuals' who disagreed with the supreme leader."
The White House responded to Flake later Wednesday, as press secretary Sarah Sanders accused Flake of serving "as a mouthpiece for the oppressive Cuban government" on a recent trip there.
"He's not criticizing the President because he's against oppression. He's criticizing the President because he has terrible poll numbers and he is looking for some attention. I think it's unfortunate," she said at Wednesday's press briefing.
Republican National Committee Chairwoman Ronna McDaniel slammed Flake's remarks on Twitter, writing, "Sen. Flake, turn on the news. It's wall-to-wall with biased coverage against @POTUS. He has every right to push back. Comparing the leader of the free world to murderous dictators is absurd. You've gone too far."
Flake, who announced he will not be seeking re-election in 2018, has said he will use his remaining time in the Senate to speak out against the President when he believes it is warranted.
Flake announced his decision to retire in a Senate speech
in October that bemoaned the "coarsening" tenor of politics in the United States and criticized his own party's "complicity" with Trump's behavior.
The Arizona Republican has said he doesn't have any formal plans
to run for President after his time on Capitol Hill.
"I don't rule anything out, but it's not in my plans," Flake told ABC's "This Week" last month.