That's not what was argued. The argument was the racist southern voters all shifted to the GOP in 1964 and stayed there.

No....the initial argument raised by LurkerMom was to mention that opposition to the Civil Rights Act of 1964 was mostly led by Democrats. LM was the one who was making that "very lazy thing to assume that people don't change culturally over time."

No lefty ever squares the circle with the fact that Eisenhower was the first President to push for broader reform and the CRA passed with more Republicans than democrats....

You need to talk with more lefties. Almost every lefty that's interested in Civil Rights is aware of Southern conservative opposition to the bill, the existence of the Solid South, and George Wallace's siphoning off of the Dixiecrats in order to try to preserve segregation. ♫♫"In Birmingham they love the gub'nor...."♫♫

In the 1950's, when Eisenhower pushed for broader reform, the parties were nowhere near as rigidly aligned on the liberal-conservative spectrum on social issues that predominate today. There were lots of liberal Republicans, and lots of conservative Democrats. Opposition to the Civil Rights Act came mostly from conservatives, while support for it came mostly from liberals. And those two ideological groups sorted fairly steadily after the passage of the CRA into GOP and Democrats, respectively. Quickly in the national races and more slowly in the state and local races. The Republican party of 1964 was far more liberal than today's GOP; the Democratic party of 1964 had a very large bloc of conservatives in the South.

If you analyze the Civil Rights Act under the political spectrum that the two parties ended up sorting on, it is fundamentally a liberal piece of legislation. You can see that if you try to identify which modern party's ideology is more supportive of the core concept of the Civil Rights Act - the use of federal power to restrict private parties and local political organizations in order to protect the interests of historically disfavored minorities.