The president on Saturday commented on the proposal, which was first reported by The Washington Post. "If they feel there will be separation, they don't come," Trump said.
"Is the president making a mistake here?" Stephanopoulos said.
Flake responded, "Yes, he is. We shouldn’t bring that policy back. That, that simply is un-American, and I think everybody recognized that. The president seemed to. Certainly the first lady and others spoke directly against it."
In an exclusive interview with ABC's Tom Llamas last week, first lady Melania Trump said the administration's earlier family separation policy was "unacceptable."
"It was unacceptable for me to see children and parents separated," the first lady said. "It was heartbreaking. And I reacted with my own voice."
Stephanopoulos asked Flake if he agrees with commentator George Will's call for conservatives to vote for Democrats in the midterms as a check on Trump.
The Arizona senator, who will leave office in January after choosing not to run for re-election, said he hopes Republicans can be a check on the president while still holding a majority in Congress.
"I think that Republicans need to provide that check, and that's what our system of government does, and I hope that they can without a change in who controls Congress," Flake said. "But I think that that may be coming if you believe the conventional wisdom, and I kind of do: The House will likely flip [to Democratic control]; the Senate will remain in Republican hands."
Flake, a Judiciary Committee member, had demanded an investigation of allegations that arose during the confirmation process of sexual assault and sexual misconduct by Kavanaugh during high school and college. After the FBI conducted a supplemental background investigation of the nominee, Flake joined the Senate majority in approving Kavanaugh for the Supreme Court.
"I know a lot of people wanted [the supplemental investigation] to be more broad," Flake said. "I would have liked to have started it earlier and had it more broad, but it was a good investigation. And I think we’re in a better place than we would have been" without the additional investigation.
Flake added that he doesn't think Congress should hold any additional investigations into Kavanaugh or into how Ford's allegation became public.
Ford's said he had that while the confirmation was an "awful process," he doesn't believe that Congress should conduct any further investigations to Kavanaugh nor the leak of Christine Blasey Ford's letter.
"I don't think that we should move forward, the Congress certainly, with additional investigations," he said.