1/2

A couple of ways. Some Confederates just bite the bullet and argue that the Founders are wrong about stuff and that their counterrevolution of treason in defense of slavery is going to perfect things by fixing their errors, but that is a minority view. More often the Confederates continue on largely as they had done before the war in pointing out that the Founders were not especially critical of slavery and, in fact, were far more often enthusiastic boosters for the process. The diversity of Confederate rhetoric on the point of whether they are doing a revolution against or continuation of the old order is something I've addressed in a prior post. But that's not quite your question here, so let's dig in.

The most common form of this is to accuse the North of breaking a sacred compact upon which the Union was built: the preservation and expansion of enslaving. On this point, it's hard to argue that they were wrong. For essentially the whole of American history to 1860, any time a great controversy arose over the expansion of enslaving, the enslavers got just about every major item on their agenda. In exchange, those who disliked enslaving and wished it would end got largely empty promises for future concessions. By 1860, the biggest issue in the rhetoric is the Republican party's declared platform of, essentially, no new enslaving states being admitted to the Union. Or more technically, enslaving is illegal within all territories ab initio and cannot be voted in by a territorial government. Furthermore the Congress voting enslaving into a territory is unconstitutional. Therefore, enslaving can only be enacted by positive law by a state government...which having been previously constituted as a free labor jurisdiction is exceptionally unlikely to ever do so. So no new enslaving states.

The Republicans of 1860 would say that's the will of their Republican fathers, by which they mean the founders of the Republic who 100% totally for sure would have joined the 1860 Republican Party if they'd still been alive, and they stand for the preservation of the Republic as it stands and as was intended. This is probably something they believed, but it's also the kind of thing you say when you have a new party trying to establish the legitimacy of its platform. A look at what the founders actually did, vis-a-vis slavery in the territories, is rather less likely to provoke outbursts of patriotic enthusiasm.

The nation's first antislavery law, the Republicans of the time and history textbooks at least up through my days in school would tell us, was the Northwest Ordinance. It's older than the Constitution, passed by the Confederation government to manage the governance and colonization by whites of the vast territories beyond the Eastern seaboard assigned to the US at the conclusion of the Revolution. Or rather, the part of that territory north and west of the Ohio River. This happens to be where I'm writing from and was, into Lincoln's time, understood as "the West." It concludes with this provision:

Art. 6. There shall be neither slavery nor involuntary servitude in the said territory, otherwise than in the punishment of crimes whereof the party shall have been duly convicted: Provided, always, That any person escaping into the same, from whom labor or service is lawfully claimed in any one of the original States, such fugitive may be lawfully reclaimed and conveyed to the person claiming his or her labor or service as aforesaid.

That seems relatively straightforward: no enslaving, but if an enslaved person escapes to the territory they must be returned to their enslaver. It doesn't hold up on closer examination, both of the facts on the ground and of the rest of the document. The ordinance was not submitted with Article 6. It got tacked on as an amendment. The reasons for doing so are murky now, because records of the relevant debate don't seem to have survived. Thanks, guys. That's odd, but what's odder still is that elsewhere the ordinance refers to "free male inhabitants", which necessitates that there also be unfree male inhabitants. The law recognizes slavery as extant at least in potential in the same jurisdictions where it is prohibited. That the text was not reconciled with its final section is not a reassuring measure of the seriousness with which the Confederation Congress took the cause of human freedom.

It's, furthermore, not just that the law imagines slavery as existing in the Northwest Territory. Slavery DID actually exist there. A fair number of French traders and trappers who lived in the territory enslaved people. They had their property rights guaranteed in the Treaty of Ghent and, on seeing the text of the Northwest Ordinance, had rather intense concern about those rights being abridged insofar as they entailed property in other human beings. They lobbied the new territorial government and were given assurances that they could keep on enslaving. Those assurances, and the lobbying, also reached the national government. Congress gave an answer that the provision did not touch French enslavers. Nor did President Washington raise an objection when he was informed that that interpretation prevailed on the ground in the territory from its governor. What is sold in patriotic textbooks as a slavery ban clearly was not but was, at most, a ban on introducing more enslaved people to the territory.

The Founders did not cease to cover themselves in ignominy there, though; they had other and less northwesterly territories to organize. The Southwest Ordinance, covering modern Tennessee, was a carbon copy of the Northwest except for the alleged antislavery provision. Nor could the Founders be exercised to attempt a ban on enslaving in the future Alabama and Mississippi, both of which were subject to a boundary dispute with Spain and the local enslavers threatened to make their allegiance to the Spanish Crown if the United States made an issue of it. The US did not. The new government lacked a realistic means to do so, but also could not be much bothered to try. Instead we have a direct capitulation in the full knowledge that enslaving will spread and the absence of a provision necessarily entails its presence.

We may do still worse. The Constitution granted enslaving a special status above that of other rights. If an enslaved person fled to a free jurisdiction, then whether or not the jurisdiction returned them previously depended mostly on the jurisdiction and situation. With the Fugitive Slave Clause (distinct from the later act of 1850), the Founders rendered slavery extraterritorial and created a constitutional obligation for fugitive enslaved people to be returned to slavery. No state emancipation law could apply to an enslaved person from elsewhere, but enslavers from elsewhere could instead carry their enslaving law with them to seize people from freedom.

And worse still: The infamous 3/5 clause granted enslavers extra representation in the Congress and extra voice in the election of presidents specifically because and to the extent that they enslaved. None of the representatives elected to seats created by that clause or electoral votes cast pursuant to it represented a single Black American. Rather they are the only ones that could never even possibly do that. (Free black men who could meet property qualifictions were a distinct minority, but some could and did vote and so might be considered represented otherwise.)