John F. Kennedy.
It’s no more complicated than that.
I had a co-worker tell me he just did the Catholic thing and voted democrat.
The Catholics have always been married to the democrat left, before JFK they had either a permanent majority democrat vote, or they may have had a slight majority support Eisenhower in 1956 as an incumbent, it depends on which polling you use.
So at most, the Catholic vote had gone GOP once, before JFK.
That is why my in laws were Democrats for a long time.
My mother in law is no longer one, only because of abortion.