This is somewhat true, although I might mention that times have changed. What is not true is that the south voted for the Democrats before the civil war - they voted for what were called the “Southern Democrats” and they were socially conservatives led by John C. Breckinridge. It may be true that the Democrats and the KKK were in cahoots, but this all goes down to how we perceive things. Both the Deep South and the more liberal states of the north voted for the Democrats at one point because back then Americans were more concerned with specific policy rather than that one party is authouritarian right and the other libertarian left, which wasn’t the case anyway. The problem is that American politics has become so incredibly divided and partisan - FDR and Ronald Reagan both carried at least 46 states at one point because of their policies - that you’re right, we have to perceive one candidate as left wing and the other as Conservative.
So what it all comes down to is that if you said this about the Republicans today nobody would be surprised, but if you said it about the Democrats we’d be in shock, because both sides see their party as principled and infallibly conservative or liberal so they don’t mind their parties going to the extremes of conservatism or liberalism respectively, just so long as they don’t become liberals or conservatives respectively.
It’s saddening to hear that a party I partially support (as a Canadian I fortunately have a third party to choose from) has been associated with extreme racism, but I support their policies of today, not yesterday.