"are Catholic" is so fuzzy though. "identify as" and "practices" are wildly different experiences, at least on the receiving end. I'm sure that percentage is mostly consisting of "identify as" rather than "practicing".
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.
At this point I don't think anything other than the church retains the ability to present a coherent moral or metaphysical intellectual framework to people who care about that kind of thing.
I would be very surprised if the united states is not majority catholic in ~100 years