If Mitt Romney runs against Trump in 2020....

[136or142]

(Assuming Trump is still in the White House)

...would Romney clear the field as the only candidate against Trump?

If Romney doesn't run, I think all of these people would run against Trump:

1.Marco Rubio
2.Rand Paul
3.Ben Sasse
4.John Kasich
5.Ted Cruz
6.Mark Cuban

If Kasich doesn't run, there are a half a dozen or other moderate or so-called moderate Republicans who could run in his place.

I think Corker and Flake will end up endorsing either Rubio or Kasich (or whichever moderate Republican runs.)

A one on one race between Romney and Trump would require a lot of popcorn.

[136or142]

Neither Cruz nor Rubio is going to run against Trump again except in the unlikely event that (post-Dems taking over Congress) Trump tacks so far left that conservative media like Fox News turns against him.  They're both waiting until 2024 to run for president again.

OTOH, I can actually imagine Rand Paul deciding to challenge Trump, despite all the sucking up to Trump that he's currently doing.  It's not likely, but it seems more likely than either Cruz or Rubio running.  Cruz and Rubio actually have ambitions to lead the party post-Trump, whereas Paul is never going to be able to do that, so he has less to lose.

I think you make a very good point, but if it seems clear that Trump would lose renomination I can't imagine either Cruz or Rubio choosing to remain on the sidelines. 

[136or142]

I don't see Sasse or Cruz running at all. Rubio and Paul might be possible, but I wouldn't bet on it (Paul seems more likely than Rubio though). But I think Kasich is very likely to mount at least some campaign against Trump.

... Maybe Nikki Haley?

Nikki Haley is one of the half a dozen other moderates or so-called moderates I mentioned.  I didn't post their names because I didn't want to add clutter or confuse.

I think the other moderates... who could run are:
1.Susan Collins
2.Larry Hogan
3.Charlie Baker
4.Brian Sandoval
5.Evan McMullin

[136or142]

He would clear the field IMO. but I think his best case scenario is probably winning Utah + the North East and DC. Virginia and Maryland would be close as well in his absolute best case scenario.

As to Romney

Trump carried the northeast in the 2016 primaries.  Trump is actually a moderate.

Since I believe Kaisch will run, I’d not believe Romney would run and split the anti Trump vote.

Romney would also be 73. Good grief Kasich will be 68.

Trump will be 74.

Trump sort of campaigned as a 'moderate' (he mostly campaigned promising different conflicting things to different groups.)  He certainly hasn't governed like a 'moderate.'