Washington CNN  — 

On Sunday, Sen. Jeff Flake, R-Arizona, was asked whether he thinks someone should challenge President Donald Trump in the 2020 Republican primary.

“Yes, I do. I do,” Flake told “Meet the Press” host Chuck Todd. “I mean, 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.”

Who could that someone be? Obviously, Flake, who was all-but-forced into retirement after his attacks on Trump led to an open revolt against him by the base of the party, wants it to be him. (He has been very coy about his own future political plans.)

And he’s certainly on the list of potential Trump primary challengers. Here’s my ranking of the top three – in order of most likely to least likely to actually run.

1. John Kasich

The Ohio governor was one of a handful of major establishment politicians who never threw their lots behind Trump in 2016. And he has been open about his interest in running again. “I honestly don’t know what my future is,” Kasich told CNN in January.

2. Jeff Flake

He’s young (55) and has a very conservative record – particularly on debt and spending issues where Trump has totally abandoned Republican orthodoxy. Even so, this feels more like a race of conscience for Flake rather than a race he can actually win.

3. Mitt Romney

The two-time presidential candidate is very likely to win the seat of retiring Sen. Orrin Hatch, R-Utah, this fall. His return to the political arena – coupled with Romney’s outspoken criticism of Trump during the 2016 campaign – has people speculating that a presidential primary bid might happen. I don’t see it. 

The Point: Beating a sitting president in a primary is a near-impossible task. (See Kennedy, Ted in 1980.) And while Trump’s numbers are historically low among the broader populace, he remains quite popular among Republicans. In short: One of these three men might run, but beating the President is something else entirely.