Donald Trump looks for reelection as an incumbent president in 2020. Normally, incumbent president don't get strong challengers from inside their own party (last exception was Jimmy Carter challenged by Ted Kennedy in 1980). Trump, however, has an unusual low approval rating and appears a weak standard bearer for Republicans. Younger politicians might not want to risk their career by running against an incumbent, but there are several reasons for a Romney run in 2020:
1. Romney was not been defeated and humiliated by Trump in 2016.
2. Trump's record has been weak or bad in the first year, Romney has more to show for.
3. Many Republicans are unhappy with Trump and wish for a better candidate. That feeling could become much stronger if Republican legislators lose big in the 2018 midterms.
4. Romney is a vetted candidate and ran twice, once he was even nominated, in 2012.
5. Romney is younger than Trump, but will be 75 by 2020, this is his last chance.
PredictIt has Romney already in the list for the Republican nominee, but running is the natural step before the nomination. I see him running, but probably not winning.