Sen. Jeff Flake (R-AZ) on Sunday said a Republican who shares his critical views on President Donald Trump should challenge him for the GOP nomination in 2020.

“The Republican party is the Trump party right now, but that’s not to say it will stay that way,” the retiring GOP senator said, acknowledging that it would be difficult to defeat a sitting president in a primary.

Amid an intensifying spat with Trump and some members of his own party last year, Flake announced that he would not seek re-election to the Senate. Flake has fueled speculation that he would challenge Trump in 2020, most recently after he accepted an invitation to speak at an event in New Hampshire, the key first-in-the-nation primary state, later this month.

“As a Republican who believes in free trade, limited government, economic freedom, I couldn’t be re-elected in my party right now,” Flake said on NBC’s Meet the Press. “Somebody who voices reservations about where the president is or criticizes his behavior like last night, it’s tough to be re-elected in a Republican primary.”

Flake, who plans to introduce legislation aimed at nullifying Trump’s newly imposed tariffs on steel and aluminum imports, said it is “tough to make the case right now” that the Republican party is still the party that favors free trade. Trump’s long-standing protectionist views on trade have caused the Republican party to begin to shift toward him—even as GOP lawmakers voice serious concerns about the tariffs and the process by which exemptions would be created.

“Tariffs are awful. Tariffs married to uncertainty is probably even worse,” Flake said.

“To have the president in a position to say, ‘Australia, alright, what are you going to do for me?’ Or wake up one day and say, ‘let’s impose more tariffs here or there,’” he added. “That’s an awful situation to be in where one person is basically deciding, tariffs go up or down depending on what kind of behavior? Something else he doesn’t like? It’s just not the way to do business.”

But privately, Republican aides and lawmakers doubt that they could reach the necessary 67 votes in the Senate to override a presidential veto on scrapping the tariffs. And it’s not clear whether Senate Majority Leader Mitch McConnell (R-KY) and House Speaker Paul Ryan (R-WI) would support Flake’s approach, as opposed to working directly with the administration to narrow the new policy.

Both leaders issued statements critical of the new policy when it was announced on Thursday, but neither mentioned the possibility of a legislative response.

Flake has said that Congress should not be “complicit as the administration courts economic disaster” through the imposition of tariffs that Republicans worry could upend the positive economic message they are hoping to use for the 2018 midterm elections on the heels of the tax-cut legislation.

Other Republicans, such as Sen. Orrin Hatch (R-UT), the chairman of the Senate Finance Committee, have said that the tariffs “fly in the face of” the tax legislation. While Hatch believes Congress will nullify that tariffs, Sen. Ron Johnson (R-WI), who has been among the loudest voices in opposition to the new policy, said on CNN’s State of the Union that he doubts whether Congress would have enough votes to override the president.