In my reading, it's not just in public discourse. Theres a genuine, felt anxiety. Americans don't know how they'll handle the complexities of being one player among many, multilateralism. It's too deeply ingrained in their thinking to be one, and the right one at that, of two (or one). We came into international power as the decisive and good military power in the morally unambiguous WWII. Through the Cold War, we were the one whose power checked the evil Soviet empire. From 1989 to 2001 we felt like we were the only ones with any power to speak of. Through all of this time our economic power was linked to our military power, and we sensed it to be the case: the military-industrial complex was a good thing.
Now who knows where the next threat is coming from. And our last war wasn't morally unambiguous. And maybe the threat is economic, and we're now losing because of our military spending. Were just psychologically unprepared for multilateralism; we dont know how well do that well, and we sense (Friedmans World is Flat) that our power might slip away quickly if we dont have our old best-of-two framework of global domination.
That may well be a version of megalomania, I don't know. But I think it's why we can't easily stomach a drop in our international power.
USA#1 isn't just a jingoistic slogan. If we can't be #1 we don't know how to be USA.