No, the founders who raped their slaves were not Democrats. The founders had a "Democratic-Republican" party, which is also referred to as "Jeffersonian Republicans" or "The First Republican Party" and isn't the Democratic party, and the other party at the time was the Federalist party.
Each of the amendments started out with the decision that the intent of the founders wasn't going to matter any longer. Any future amendment must do so as well.
Democrats fought to keep slavery, and they fought to prevent women from voting.
Well, that's really bad. But the Democrats wisely decided to stop doing those things. In the years that the Democrats cut their ties with the segregationist portion of Southern voters, spanning from the Goldwater to the Nixon campaigns, the Republicans took them up. So we're now in the position that the Republicans are the political heirs of the 1964 Democrats. So having taken over the bad stuff the Democrats used to do, you are not in a good position to revile us for our past sins.