Probably a loss of any international clout the US ever had. Trump's pissed off everyone, but right now if Biden got in they could play it off as a fluke and recover most of their pre-Trump respect. But if they double down, it'll all vanish and no one will ever take them seriously. Why sign a treaty, if they keep electing people who break them? Why ask for help preventing your ideological gangs from running rampant when they run away the minute a totalitarian leader flatters them for five minutes?
Plus most of the country dying from the totally-a-hoax-wait-it's-real-just-overblown-wait-it's-a-plague-we-didn't-take-seriously-so-I-guess-we'll-blame-China thing.