From The Propaganda Professor:
Right-wingers, as we’ve observed, are obsessed with rewriting history to cast themselves in a more favorable light. Inevitably, this includes trying to whitewash the movement’s core of racism and white nationalism — and in the process diverting attention elsewhere and saying “no, it’s really the other guys who are racists”. In this connection, they focus on the fact that 150 years ago or so, it was the Democratic Party that championed slavery, and the Republican Party that fought to eliminate it. What they ignore is that the two parties bearing those names back then bore no resemblance to the respective parties thus named today. Actually, they don’t just ignore this; they vehemently deny it when someone brings it up. Two particular phrases, closely connected, have been the targets of their denial: “party switch” and “Southern Strategy”.
“Party switch” refers to the idea that the G.O.P. of today more closely resembles the Democratic Party of yore (particularly on matters of race), and vice versa. In an effort to poo-poo this, right-wingers have narrowly defined “party switch” as the act of having a large number of politicians literally switch parties in a short space of time. And this, they correctly point out, did not happen.
Trouble is, that’s not really what “party switch” refers to in this context. It means that the respective parties began to change their platforms to attract different kinds of politicians and different kinds of voters. And that definitely did occur.