> This is oft repeated, but what’s the evidence?

All over the place.

But probably the simplest evidence is Strom Thurmond.

One politician flipped an entire party’s platform? What about the 20 democrat senators who voted against the act and stuck with the party?

A campaign of FUD regarding states rights (the basis of democrats own secessionist arguments) and conservative policies and blindness to continuing racist liberal policies is more likely (including the current president).

> One politician flipped an entire party’s platform?

I think you need to learn the difference between saying something is a simple piece of evidence of an effect and saying it is the cause of the effect.