In short, American communists and the radical left generally have long targeted the Roman Catholic Church. They know their enemy, one that is both spiritual and eternal. They have long attempted to pit Protestants and Catholics against each other. It's an old art, really, that's today totally forgotten.
Didn't work this time. The Southern Baptist leadership came out on the side of the RCC. One said they're willing to go to jail for them.
The leftist tactics are not so much an attack on the Catholic Church, specifically, but Conservative Christians in general and, of course, Almighty God in particular.