Caution about polls notwithstanding, I don’t find it the HARDEST thing in the world to believe. Romney was banking on guiding the GOP back to something resembling sanity post-Trump, clearly, and it was only eight years ago he was the leader of the party. Trump won with a plurality, and although he definitely got the GOP base to fall lockstep behind him, a lot of that was softer support and polls consistently showed that even many people who said they approved of him would prefer someone else. His actual diehard base, those who are his biggest fans, has not actually grown THAT much since 2016, it seems.
In any case, if Romney does intend to run again, he should take this as an encouraging sign that there’s hope the GOP voters could be persuaded to move on. Keep in mind their last three presidential nominees (W, McCain, and Romney himself) have all been thrown under the bus and disavowed lately by much of the GOP. After they insisted to all of us Democrats we were anti-American for not supporting them. It’s a fickle party, to say the least. Don’t be too surprised when Trump is next on the chopping block.
Trump should certainly be concerned, too, if he can only keep a quarter of the party solidly in his camp even NOW, before his presidency is even over officially. If he can’t do better than that now, how the hell will he win the primary in four years?
Trump's gambit is to maintain constant exposure and press, the problem is there is such a thing as over saturation and Trump could easily nuke himself just by being a constant reminder of how he was and what he would be like again. Trump fatigue basically and Republicans are not immune to this either.