Probably the more important question is when the USA will have second after JFK president (although its hard to call Kennedy decent follower) who will be Catholic? I guess that will be some Democratic candidate so I guess this question is not that offtopic.
That's not really an important or often discussed topic because as someone noted Catholic/Protestant identity politics don't really exist in the US anymore. This wouldn't be an issue.
And based on demographics it'd be just as likely to be a Republican as a Democrat.