Slavery has existed as far back as history. The idea that people are created equal, and are equal under the law, is very, very new, and that idea has resulted in a great many positive things happening.
A good general wouldn't voluntarily shift the battleground to a region where their troops are weak. Political persuasion is no different. The Democratic Party keeps losing because they cling to unpopular positions on cultural issues. If Trump is as bad as you say, prove it to me by showing willingness to strategically cede ground on a few issues in order to win more voters.
What I do see is media and politicians on the Right characterizing extreme positions held by a minority of folks on the far Left as though they are adopted by the entire Left, and that weak uncritical minds on the Right believe that nonsense. Rush Limbaugh pioneered this technique to profit himself, and Trump gave him a fucking Presidential Medal of Freedom in his first term.
Maybe I should work harder to make my actual views clear. I added a few sentences to the end of my original comment in this thread: "I don't think this is a slavery thing. Trump just sucks." https://news.ycombinator.com/item?id=49572347