The problem is that people continue to use words like "left" and "liberal" as a vague proxy for party platforms, but it was never a one dimensional axis like that.
For example, twenty years ago you would expect to find Democrats arguing for "liberal" values like free speech and due process, and Republicans arguing for globalist foreign policy and military interventionism. Now it's going the other way. But which party is captured by the teachers unions and which party is captured by the oil companies hasn't changed, because it's not a one dimensional problem space.
That wasn't quite the same thing though. The impetus for the so-called flip was the Civil Rights Act.
It was becoming increasingly obvious that the Republicans were going to pass the Civil Rights Act and the racists were on the wrong side of history. But the racists made a final push to put it off and managed to put the Democrats in the majority.
Then the Northern Democrats voted with the Republicans to pass the Civil Rights Act anyway. The racists were livid. Their party betrayed them.
Nixon (yes, that Nixon) realized that it made the South his for the taking, so he took it. It was a realignment.
The Democrats like to portray this as the racists switching parties, but it was really the process of the racists losing and dying out. In 1880 the Democrats were the party of slavery and Jim Crow. In 1980 there was no party of slavery and Jim Crow. They lost.
That was the point when racism dissolved into classism. It's why Democrats insist on calling classism "structural racism" -- they've convinced people the "racists" are cardboard Republicans from the South, even though they're the ones tying schools to housing and restricting multi-family zoning in blue cities.
Not to say that the Republicans are saints. War on Drugs has been a predominantly Republican dung fire, for example.
For example, twenty years ago you would expect to find Democrats arguing for "liberal" values like free speech and due process, and Republicans arguing for globalist foreign policy and military interventionism. Now it's going the other way. But which party is captured by the teachers unions and which party is captured by the oil companies hasn't changed, because it's not a one dimensional problem space.