I’m not sure on this.
I think learning Roman history is neither better or worse than French history, and there probably isn’t enough time in high school to do both in a way that has lasting impact.
This is a guess, but is this next paragraph approximately your idea:
Except in the most heavily Democratic neighborhoods, teachers cannot teach democracy good, MAGA bad. The students, or at least the parents, will rebel. But if you teach about Roman republic vs. empire, almost everyone will agree with the republic being better than the empire, and, with luck, when a Trump comes along, educated adults will be primed to figure out on their own that MAGA is strongly in the empire direction.
If that’s the idea, well, maybe. But I think the political effect on adults will be unpredictable. Those who recall a great deal of their high school history, of any kind, twenty years later, may tend to be Democrats. But I doubt having taken Roman history will be a big predictor. If you are emotionally primed to be a Trumper, it would be easy to come up with reasons why your old teacher was a doofus lib, and that the Roman republic was as bad as the empire. You might also decide that Trump is not really an emperor (much as I think he tends in that direction).
To put it another way, who you vote for is IMHO decided unconsciously, and even though I had two semesters of college Roman history, I doubt my unconscious examines that part of my memory store when making my voting decisions.