All of the "progressive" icons before Kennedy were either explicitly or implicitly in favor of segregation. An exception would be Truman, who gets a lot of grief from progressives for his use of the nuclear bomb.
Yes, the Dems tended to become more sensitive to Civil Rights issues, when a third of the blacks who were allowed to vote in the north voted for Eisenhower in 1956. That scared the sh**t out of the Northern Dem bosses, etc. There just aren't a lot of saints in politics. Rather, they tend to be rather amoral, and all too often, craven, entrepreneurs. One wonders how civil rights history would have evolved absent the Chicken Bone Express. But that is a topic for another time.