If it's true, half of the adults in the USA are enemies of the other half.

they are likely nearly always to be, now that politics has fully nationalized. 50/50 is a structural attractor. a party getting much less than 50% will steal positions from its opponent, like Trump Republicans did, until it’s back to near even. www.interfluidity.com/v2/7687.html

100% Maybe I should have said "it's wild that they haven't had to broaden their coalition beyond that to be viable"

Those are not the current numbers.