I know it's not what you intended, but I have some (non-offended) thoughts:

The meme isn't offensive, but it isn't funny, either. Since the parties switched sides on the racism issue in the last half of the 20th century, it doesn't really resonate. To be historically accurate, it would have to be "(The people who became Republicans) haven't been this mad since (the people who became Democrats) took their slaves away." And that's just too clunky.

Also, I find the people who are quickest to take offense and most vigorous in expressing it come from places like Alabama and Indiana. Hang a crucifix in a bottle of urine or bulldoze a confederate statue, and those guys go apeshit.

The issue is I don't think things that offend are created equal. Offending straight white Christian men doesn't bother me because those people are at the top of the social hierarchy. They should be able to take it.

However, people who aren't those things are taking constant, unfunny abuse all the time.

If a Black person makes a potentially offensive comment about white people, it's a meh situation. If every Black person in America hated me, it wouldn't impact my life much. But for me to make a potentially offensive comment about Black people, that's just one more indignity on top of all the other ones they're already dealing with.

Ditto for gay people - when someone is under constant threat of discrimination, injury, or possibly death, I can see why a joke at their expense is more than just a joke, it's a potential threat. I mean, we only have two political parties, and one is bound and determined to make them second-class citizens. They are only just making some progress at getting out of the closet, making life uncomfortable for them seems cruel.

So I think the definition changes according to the socio-economic stations of the parties involved. Punching up is funny, punching down is offensive.