What is prejudice? To anyone who is etymologically and linguistically sane, the definition is very clear: it's based on the prefix "pre-," meaning "before," and the root word "judice" which is just a fancy word for judging. It literally means judging someone without thorough enough investigation. Based on this definition, the Democratic Party has never ceased being the party of prejudice, and prejudice is bigotry no matter who it's used against.
It all started when Democratic Party founder Andrew Jackson's vice president, John C. Calhoun, came up with the rather absurd idea that slavery is beneficial not only to the slave owner but also to the slave, which is indeed an absurd idea to say the least. From then until the Civil War, the Democrats — and every single slave at the time of the Civil War was owned by a Democrat (this is about slave owners, not slavery supporters, so support of slavery is a red herring in this regard) — would often assume that all African-Americans were property and give them absolutely no civil rights whatsoever. Any stray African-American was pre-judged as a runaway slave even if he or she had entered the United States from somewhere else. It was complete tyranny.
After the Civil War, however, when the GOP began to gain full control, they literally found themselves scrambling. Not only did they proceed to create a militant arm — the KKK — that perpetrated the mass murder not only of African-Americans but also of white members of the GOP, but they also often pre-judged African-Americans as sexual predators of white women to justify lynching. They went on to use this same prejudice — judging people as rapists simply because they're black — in the 20th century to justify their support for Jim Crow laws, which also did not end well.
After this period ended, the prejudice never did. It simply changed forms. How so? Instead of pre-judging blacks, they turned their attention entirely to pre-judging Republicans and other conservatives. They perpetuate the "big switch" narrative despite the fact that, A, less than 20 of the 1600 most racist of the Dixiecrats switched (thanks, Dinesh), and B, the reason why non-racist whites (in the 1980's) and blacks (in the 1930's) switched parties was because of money, not because of racism. They take the positions of people like Strom Thurmond and smear them onto the Republican Party as a whole (hasty generalization), then pre-judge Republicans in general as racist, pre-judge pro-life Christians as sexist, pre-judge Christians in general as gullible, pre-judge those who don't agree with the homosexual agenda in its entirety as homophobes and transphobes, and pre-judge fiscally conservative African-Americans as "coons" and "Uncle Tom" without doing any research whatsoever.
Regardless of whether it's prejudice as a slave, prejudice as a sexual predator, or prejudice as <enter -ism or -phobia here>, prejudice is prejudice, period. Just as Ida B. Welles taught that prejudice must die and just as the GOP of the 1860's and 1870's taught that prejudice must die, so too am I teaching now that prejudice must die. They accuse all of their opponents of being "haters" prejudicially, in spite of the simple fact that prejudice is hate and that pre-judging opponents as hateful is hypocrisy on top of hate.