Outgoing Republican Sen. Jeff Flake said he would have suffered from insomnia had he chosen to run for re-election and been pressured to support President Trump.

"I couldn't sleep at night having to embrace the president or condoning his behavior or being okay with some of his positions," Flake told the Washington Post in a new interview.

"I just couldn't do it," he added. "It was never in the cards."

The Arizona senator sent shockwaves through Washington when he announced earlier this week he would forego running for a second Senate term next fall. Flake slammed Trump and the current political climate in a floor speech about his decision.

"I knew that when I spoke out at that time that I was out of step with a lot of Republican primary voters, but I felt that I had to do it," Flake told the Post. "I had hoped – and I still hope and I'm confident at some point – that the fever will break. But it just became more and more apparent that it certainly wasn't going to break by next year."

Trump responded to Flake's retirement announcement before heading to Dallas earlier this week for a hurricane relief briefing. The president told reporters he originally thought Flake was a Democrat the first time he saw the senator on TV.

"I think he did the right thing for himself," Trump said, adding Flake's retirement would help him "greatly" in Arizona.

Polls showed Flake trailing insurgent GOP candidate Kelli Ward, who ran unsuccessfully against Arizona Sen. John McCain in 2016, before he announced his plan to leave the upper chamber.