That the issue of slavery has weak explanatory power for what is happening today.

I'm anti-Trump. If you tell people "Trump is a corrupt asshole", that has a chance at convincing supporters to abandon him. If you tell people "the US needs a cultural revolution due to events from past centuries", that's what got him elected in the first place.

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.