U.S. Sen. Jeff Flake (R-Arizona) says he hopes someone challenges President Donald Trump in 2020, and that he’s giving it some serious thought himself.

A frequent critic of the president, Flake told Greater Boston’s Jim Braude that while launching a bid for the White House is not part of his immediate plans, it could be in the cards.

“I’ve not ruled it out,” Flake told Braude in an interview Friday after delivering a speech in New Hampshire, a traditional stop for presidential hopefuls.

Last year, the junior senator announced that he wouldn’t run for reelection, citing the fact that he couldn’t win a Republican primary under the current political conditions as part of his reason for leaving Congress after nearly 20 years.

“It became clear to me that unless I was willing to condone the president’s behavior, agree with some of his policies, and agree not to criticize him — basically embrace him — there’s no path in a Republican primary right now,” he said, referring to the influence Trump has had over his party. 

Flake says he’s concerned about the direction his party is headed, and recently laid out his case in a new book: "Conscience of a Conservative: A Rejection of Destructive Politics and a Return to Principle." In it, he accused the GOP of abandoning longstanding conservative practices and policies and for choosing to overlook Trump’s repeated attacks on institutions like the press and judicial system.

“I think …if you’re a conservative or claim to be a conservative you should not just be conservative on policy but in behavior as well — and in comportment and demeanor. And the president is decidedly not,” Flake said when asked about his animus toward the president. “It makes a difference. Words matter. Behavior does matter, and that’s far from conservative.”

Flake has sided with the president in several key policy areas, including tax reform and the push to repeal the Affordable Care Act — items he said he’s voted for long before Trump. More recently, Flake split with Trump, as have others in the GOP, on approaches to import tariffs, immigration, and gun reform. 

“I think that Republicans everywhere, a lot of Republicans, are wanting to be reminded of what it means to be conservative — to what decent politics actually was, and can and should be,” Flake said. “And so I hope somebody does run in the primary, and I think that somebody will run as an Independent as well. But there definitely will be a challenge.”

To watch Sen. Jeff Flake's entire interview, click the video player above.