Guilt by Association? Or something else?

It looks like a case of going from the specific to the general. "Hasty Generalization" might be a specific fallacy here.

P1: Anything that promotes even one bad idea is bad.
P2: The Constitution promotes slavery.
C: The Constitution is bad.

Premise 1 demonstrates the fallacy.

We could show the extreme weakness of the argument using reductio ad absurdum - the Roman Empire used slaves, as did ancient Greece, Celtic tribes across Europe, and ancient China. West African nations enslaved each other and sold those slaves to anyone who would buy them. Arabian slave traders captured, bought, and sold slaves across North Africa.
Therefore, Europe, China, West Africa, North Africa and the Arabian peninsula should all be abolished.

I would call it an ad sequiter. It does not follow that the electoral college should be abolished because is supported slavery. It is question begging that the electoral college supported slavery. Maybe this is true but I need better proof.

As arguments go it's on thin grounds, with two errors in its very presentation.
Nothing about The Constitution promoted slavery. To its discredit, it acknowledged it, and that has been repaired by Amendment.
The Electoral College had and has nothing to do with slavery at all.

Don't know what happened to an earlier answer I wrote, but this is just a variation of the Hasty Generalization Fallacy, where an extreme solution is provided to remedy a single problem, such as:

"The Constitution which supposedly protected states' rights actually protected the institution of slavery, therefore the Constitution should be abolished for that reason alone."

The Constitution has built-in procedures for correction and though they can be nearly insurmountable at times, there are better ways to amend those grievances than the extreme measure of abolishment.

"The Electoral College enabled slavery, therefore, the Electoral College should be abolished."

Again, this is an extremely broad solution to a single sample. Though slavery and states' rights protections were an egregious flaw they are just one of many reasons why the Electoral College should be redrawn or possibly eliminated. Just equating slavery with the flaws of the Electoral College is a type of poisoning the well when there are many less hot button reasons for abolishing it as well.

Another example of the Hasty Generalization Fallacy might be:

America's tolerance for religious freedom is leading to a Christian dominianist theocracy, therefore the First Amendment needs to be abolished or amended.

"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof;"

There are many other steps that can be taken to balance out of control religious freedom other than abolishing religious freedoms overall.

So the object of this argument it they want both the Constitution AND the Electoral College to be abolished? - TWO CONCLUSIONS?

The first premise - is this actually the first premise? If so why does it have a conclusion in it: "Therefore"?
"The Constitution which supposedly protected states' rights actually protected the institution of slavery, therefore the Constitution should be abolished for that reason alone."

The first line is actually a complete logical argument WITHIN first premise,
1. The Constitution {which supposedly} protected states rights;
2. states' rights actually protected the institution of slavery
3. therefore the Constitution should be abolished for that reason alone.
-- then it goes on and introduces a NEW argument about the Electoral College.

There are FOUR elements in this entire argument: The Constitution, States Rights, Slavery and the Electoral College. That is too many.
So the argument is not in proper logical form. It is two logical arguments jumbled together.

There are other issues with the argument too.

The Constitution did not address or establish slavery. Slavery existed before the constitution. Slavery became an institution within the colonies which became states.

It was not an issue with states rights but an issue with definitions of what a person was. Same issue with Indians.

This argument is a total mess.

I'm not sure what to say here. Is there such a fallacy as "throwing the baby out with the bathwater?" LOL

If you think a constitution needs to be perfect, you might find this argument convincing. If you think that the constitution is imperfect but needs to be fixed, you won't find it convincing.

Maybe it's "Confusing the part for the whole?" Like this: "My spouse has a bad haircut, so we need to break up." Or something like that.

Anyway, I've heard this argument, so this is an important question. Slavery is awful, but that doesn't mean that the rest of the Constitution is bad.

This would be a better argument: "Some people think the framers of our Constitution were demigods. But they supported slavery, so they had terrible flaws, and we should not think of them like demigods. We should feel free to fix what they did." Does that make sense?

Thanks for asking. Let's see what Dr. Bo says!