"I don't rule anything out, but it's not in my plans," Sen. Jeff Flake (R-Ariz.) told Jonatan Karl about running for president in 2020. | John Shinkle/POLITICO Flake: If Trump fired Mueller, it would be a 'big problem'

Sen. Jeff Flake said Sunday that if President Donald Trump were to fire special counsel Robert Mueller, it would be a "big problem" for the president.

"I don't think that would go over well at all here in the Senate," the Arizona Republican said on ABC's "This Week." "I don't think he'll go there, he shouldn't go there."


Flake's comments come as the president's allies continue to take Mueller and the FBI to task. The White House has consistently said Trump has no plans to fire Mueller, but president joined in the bureau bashing this weekend with a series of tweets about FBI Deputy Director Andrew McCabe.

McCabe, who is reportedly retiring soon, was a key figure in the investigation of former Secretary of State Hillary Clinton's private email server.

Flake has burnished his reputation as one the president's most frequent critics in Congress, but was cagey when ABC'S Jonathan Karl asked if the senator would challenge Trump for the GOP nomination in 2020. Earlier this year, Flake announced that he would not seek reelection to the Senate in 2018.

"I don't rule anything out, but it's not in my plans," Flake told Jonatan Karl.

The Arizona senator did say he expected that if Trump runs for reelection, there will be an independent challenger of some kind. He said, "I do believe if the president is running for reelection, if he continues on the path that he’s on, that that’s gonna leave a huge swath of voters looking for something else."