I don't think supporters of the current US president would be horrified. They already support his anti-christian behavior, and seem more interested in being part of that group than they are in the religion itself.

More likely he would just assert that the Pope isn't actually the Pope, and thus any excommunications are void, and his supporters would roll with it. Some of them already believe this. Any words, true or false, which make them feel better to believe. That's religion, right? He is their true religion.