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Sen. Jeff Flake, a frequent critic of President Trump, said the New Yorker's rhetoric and governing style may invite a Republican primary opponent in 2020.

Flake (R-Ariz.) did not rule out running against Trump himself, in an interview with ABC's Jon Karl.

"I do wonder and I do worry that in the future, we'll be faced with President Trump running for reelection on one side, drilling down hard on a diminishing base, and on the other side you might have [a far-left candidate] like Bernie Sanders or Elizabeth Warren.

Flake said such a phenomenon would leave a "huge swath" of voters "in the middle" without a defined candidate.

"Go for it dude," Jones told Lea Gabrielle. "You can't even win a primary in your own state. That's why you're not running for reelection right now."

Flake is retiring in 2018.

"This is a guy who votes for Democrats and sends money to Democrats," Jones said of Flake, who published evidence of a check he sent to Sen.-Elect Doug Jones (D-Ala.) in protest of conservative Judge Roy Moore.

Jim Kessler, a former aide to Sen. Charles Schumer (D-N.Y.), said Trump's approval rating "could freeze water" and that Republicans should be concerned about 2018 and 2020.

He said Virginia Gov-Elect Ralph Northam (D-Norfolk) was the moderate primary candidate against liberal former Rep. Tom Perriello (D-Ivy).

Kessler said Democrats are already starting to shift to the middle in elections to combat the more conservative Trump.

"If you give [Trump] a primary challenger, he's going to be fired up," Jones warned, saying the president can galvanize support with raucous rallies.

Watch more above.

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