At the beginning of the 20th c., the GOP was very hostile to Catholicism and to the flood of largely Irish and Italian immigrants, many of whom were Catholic. When Democrats nominated the first Catholic presidential candidate in 1928, the GOP responded with anti-Catholic ads like this one.
The hostility of the GOP to Catholics started a multi-generational devotion to the Democrat brand that reached its peak with our only Catholic president, JFK.
All of this predates the Democratic Party jumping on board the abortion and same sex marriage band wagons, but brand loyalty is a tough nut to crack.
Having been raised in an all-Democrat environment, I experienced this difficulty personally. I worked on Jimmy Carter’s campaign, but felt compelled to leave the party after the DNC abandoned the unborn. For years, I hunkered down in the Right to Life party, voting Republican only because they were the most pro-life candidates, but never, ever identifying myself with “those people.”
When Right to Life folded in New York, I got a letter advising me to register Republican. (NY primaries are open only to registered party members.) As I read it, my stomach heaved like I was car sick. I realized it was a visceral reaction to the IDEA of registering Republican. Until this happened, I had NO IDEA I harbored this powerful prejudice!
Add this inherited revulsion for the GOP brand to the fact that lots of people are just too busy to research political issues beyond what the MSM and Democrat Catholics like Pelosi and Biden tell them. And why should they when they get so much support for the “Democrats are the goodies, Republicans are the baddies” reality they were raised with? I pray for them every day.