"We've gotten nine months into the administration. Those of us who had hoped for a pivot, I think, agree now it's just not going to come, and so it's up to us to stand up and say this is not acceptable," Flake said on "GMA," echoing his speech on Tuesday.
Sen. Bob Corker, another Republican who's retiring, called Trump "utterly untruthful" and accused him of "debasing" the United States just before Flake took to the Senate floor to say he would not be "complicit or silent" in regard to Trump and his behavior.
Trump later attempted to shoot down Flake's prediction that more of his colleagues will speak out against the president, tweeting that GOP senators gave him a "standing [ovation]" when he met with them on Capitol Hill on Tuesday.
But Flake said Congress should "certainly" act if Trump follows through on some of the threats he has made. He cited Trump's threat to revoke television network licenses — which critics have argued is a threat to rights guaranteed by the First Amendment.
Flake argued that his colleagues must also "speak out against the tweeting and the behavior that just isn't becoming and is beneath the office of the president."
He didn't rule out challenging Trump in 2020 for the Republican presidential nomination. "You know, that's a long time away. And we'll deal with that when it comes to it," Flake said.
With 14 months left in his term, he said he plans to focus on pushing for an authorization for use of military force against terrorist groups and finding a solution for enrollees in the Deferred Action for Childhood Arrivals immigration program, which the Trump administration announced last month it will wind down.