I figure 27% is the lower bound on public approval of a Republican president/presidential candidate (+/- a bit). I'd say 38% is the lower bound on voting for one, absent a major 3rd party candidate confusing things. That's just a measure of tribalism in action. Those people don't actually have to like the candidate, they're just going to pull the lever for the R no matter who it is. My measure for election day is how much above 38% Trump gets, otherwise known as the How Fucked Are We measure.
38% is just something I made up, of course.