That's independent of the EC. They could have given the slave owners 3/5ths of a vote for each slave without the EC. And obviously that part of the system is no longer in operation, whereas the part Democrats complain about is that each state gets +2 electoral votes regardless of its population.
Which nominally gives slightly more weight to the lower population rural states, but that isn't even the primary consequence of the EC. The primary consequence is that it gives significantly more weight to swing states, which by definition don't favor any given party.
> They could have given the slave owners 3/5ths of a vote for each slave without the EC
Yes, I suppose if you could accept the idea of a ludicrous hypothetical alternative that would have zero chance in reality of being implemented you can contort yourself enough to ignore that the EC is part of the compromise on slavery that forms the Constitution.
It's ludicrous by modern standards because the premise of owning other people is ludicrous by modern standards. Giving states more votes based on them having people there who can't actually vote is exactly the same amount of ludicrous, but that's also the part that isn't there anymore.
The primary thing the electoral college does in modern day is allow -- not even require -- states to allocate all of their state's voting power to the candidate that wins the majority of the state. With the result that they mostly do that and then states like New York and Texas get ignored in Presidential elections because nobody expects them to flip and getting 10% more of the vote is worthless when it doesn't flip the state.
Ironically it's the partisans who are effectively disenfranchising the people in their own state. If the states that go disproportionately for one party didn't want to be ignored then all they'd have to do is allocate their electoral votes proportionally according to what percent of the vote the candidate got in that state. Then getting 10% more of the vote in a big state would be as many electoral votes as some entire states. But the non-swing states are by definition controlled by one party and then they're willing to screw over their own population to prevent the other party from getting any of that state's electoral votes.
Which nominally gives slightly more weight to the lower population rural states, but that isn't even the primary consequence of the EC. The primary consequence is that it gives significantly more weight to swing states, which by definition don't favor any given party.