Former Sen. Jeff Flake (R-AZ) says he will not be casting his vote for President Donald Trump in 2020, citing concerns he had with the president before the 2016 campaign.

“I just couldn’t support [Trump] long before he started to run. The birtherism thing was just too much for me. And then it piled on,” Flake, a vocal critic of Trump, told the Washington Post.

Flake also warned that a second Trump term could deter younger voters from the GOP, especially on issues concerning the environment and immigration.

“So for young people who’ve grown up around minorities or had a different experience than a lot of us in my generation, they don’t harbor, I think, some of the prejudices that people in my generation do,” Flake said.

Flake added that a Trump defeat in November would benefit the Republican Party and conservatism in the long run.

“This won’t be the first time I’ve voted for a Democrat — though not for president. Last time I voted for a third-party candidate,” he said. “But I will not vote for Donald Trump.”

Flake also predicted the GOP would change once Trump ends his time in the Oval Office.

“The pendulum swings when one party takes it too far,” he said. “We’ll be ourselves again.”

Flake served in the Senate between 2013 and 2019 and served six terms in the House before that.

As a senator, he criticized Trump on a variety of issues, including his negative comments on his colleague, former Sen. John McCain (R-AZ) in 2015.

Politico reported that Flake was also “shocked” to discover that his friend and former House colleague Rep. Mike Pence (R-IN) was selected to be Trump’s running mate in 2016.