Flake has distinguished himself as one of Trump’s most vocal Republican critics. He previously endorsed Joe Biden for president in 2020, and served as the administration’s ambassador to Turkey, before resigning in September. The Arizona Republican issued his official endorsement of Harris on Sunday morning, appearing on ABC News’s “This Week” shortly after to explain his reasoning, and why other Republicans should join his lead.

“Well, I think Republicans believe in the rule of law in particular, and it’s difficult to support a candidate who, having lost an election, tries to use the powers of the presidency to overturn that election that is anything but respect for the rule of law,” Flake said. “So I know that a lot of conservative Republicans feel the way I do, that you just can’t support a candidate like that.”

On terms of foreign policy, he said that Harris is “ready,” and “understands what’s going on.”

“She understands that we have real enemies abroad, but in particular, we have indispensable allies as well, and we need to support them and work with them if we’re to defeat Russia, certainly, and the growing threat of China,” Flake said. “It was really stark watching the debate the other day and hearing the former president not be able to even cheer for Ukraine.”

Host Martha Raddatz then pressed him on immigration, saying that polling showed Harris’s stance was “simply not resonating with voters.” She asked how Harris could change voters’ minds before election day.

“I think she’s doing that,” Flake responded. “And she outlined a couple of policies where she would be more, she would be tougher on the border, particularly with asylum policies. We’ve got to have a situation where we have comprehensive reform.”

“One thing that she brings to the table is that she knows how to work on a bipartisan basis, and if we do immigration reform that endures, it’s going to have to be bipartisan,” he continued. “So I’m obviously very pleased that she would sign the legislation that was worked up in the Senate. That’s what we need to do. It needs to be bipartisan in order for it to be lasting.”

Flake served as an Arizona senator from 2013 to 2019, choosing not to run for reelection after consistently low approval ratings.

He could be vying for a Cabinet position, a possibility floated by Politico. He was appointed by Biden to be ambassador to Turkey in 2022, a post he left this month. Harris previously vowed to appoint a Republican to a Cabinet position.