This is pure spin.
The civil rights legislation began with two bills singed by Eisenhower late in his administration, followed by several landmark bills signed by Johnson. All bill were passed by a Congress in which both houses were controlled by Democrats. Lyndon Johnson was the Senate leader during the last years of the Eisenhower administration. He led these bill to passage, knowing full well the effect it would have on the party’s electoral prospects. Strom Thurmond then a Democrat, afterwards a Republican, filibusterd against the 1957 bill. After Eisenhower, the Democrat held fiibuster-proof majorities. Southern Democrats continued to resisted. Overtime, since that era, the deep south, once solidly Democratic, became solidly Republican - apart from the first Cater election. I don’t doubt that it was, as you suggested, a reaction to progressivism - things like equity in voting rights, housing rights, educational opportunities, etc.
Through the FDR years, Democratic party affiliation among blacks was steady at ~42%. During the Truman and Eisenhower years it jumped, but for the most part remained steady at about 56%. In 1964 it had jumped to 82%; in 1968, 92%. That era of hard-fought and hard-won landmark legislation was clearly the turning point that ushered in the era a Democratic party affiliation among blacks in vast majorities.
Your narrative is not compatible with the facts.