After his military service, Reagen returned to Hollywood and was agast at the communist presence. From those early days he was a big fighter against the communists in America. That is why he was hated so by the democrats.
I think checking the record will reveal that Reagan remained a democrat for quite a few years, after WWII.
Scoop Jackson was a democrat who was strongly against communism. I don't think the democrat party veered away from American patriotism very much, until the Vietnam era.