Sen. Jeff Flake (R-Ariz.) says President Trump should face a Republican primary challenge in his 2020 reelection bid.

Asked on NBC’s “Meet the Press” whether he thinks Trump needs to square off against a Republican who espouses Flake’s more traditionally conservative views on free trade and more, the retiring senator said, “Yes, I do. I do. It would be a tough go in a Republican primary. The Republican Party is the Trump party right now. But that’s not to say it will stay that way.”

Flake, one of the GOP’s most outspoken Trump critics, is stoking speculation about his 2020 plans with a Friday appearance in New Hampshire as part of the “Politics & Eggs” speaker series, which draws presidential hopefuls to the first-in-the-nation primary state. Since announcing his retirement in October, Flake has launched speeches on the Senate floor questioning Trump’s fitness for office and calling his behavior “dangerous to our democracy.”

Flake is also planning to introduce legislation to nullify Trump’s tariffs on steel and aluminum imports, set to take effect later this month. The president rolled out the tariffs Thursday, extending an initial exemption to Canada and Mexico while raising the possibility of exemptions for other countries that renegotiate their trade policies with the United States. “The problem is when you say, ‘Let’s have tariffs, but let’s couple that with uncertainty’ — that’s almost worse,” Flake said. “Those are dual poisons to the economy.”

The Arizona Republican said it would be “tough to make the case” that the GOP is still the party of free trade. “I think we’re going completely the wrong direction,” he said.