What were the main (flawed) arguments for keeping slavery legal in the US?

Proslavery advocacy comes in many forms. Conventionally we separate them into Necessary Evil and Positive Good schools, the latter rising to prominence in the 1830s. That's a good first approximation, but it implies that the two are more different than a close examination can support. It also neglects that both streams of thought exist from roughly the time that American slavery is first under serious, if infrequent and moderate, criticism.

So let's go into them. The Necessary Evil argument is probably the dominant form of proslavery discourse until around 1830, though like I just said it's never really alone. The argument holds that slavery is immoral and wrong in the abstract -the Evil- but ending it is impractical for a wide variety of reasons and therefore Necessary. Those include no state having the money to just buy all the enslaved people their freedom, an inability to remove freedpeople from the continent meant for white men alone due both to costs and not being able to remove enslaved people faster than they reproduce, the staggering financial and social costs to enslavers, and so on. Most Necessary Evil people will say that they want slavery gone at some time in the future. They will further insist that enslaving can only cease with the full and enthusiastic consent of the enslavers...who happen to be making piles of money out of their regime of torture. Thus we should read that future date as, effectively, Quarter to Never.

This is all important to keep in mind because Necessary Evil people are easily misunderstood, even by their contemporaries and perhaps by themselves. We tend to read it with emphasis on the Evil and gloss over the first half. It's really a matter of the other way around, in practice. They find slavery absolutely necessary and have no particular interest in ridding themselves of the evil, which is central to their civilization and their pocket books. The common misreading leads to absurdities like insisting enslavers are antislavery even when they're literally arguing on the floor of Congress that slavery must be allowed to spread and grow, an amazing error of reasoning that is congenial to patriotic myths about white Americans of a certain age who have their signatures on documents on display at the National Archives that we make at considerable expense to our ability to understand the past. People at the time can be forgiven that, since they lack hindsight and the appearance of a consistent apology for slavery rather than full-throated defense seems more plausible in an era when enslavers were deprived of their human property legislatively, at least in the North.

The Positive Good school is having none of this. They're willing to own up to slavery being something they want to retain indefinitely and expand broadly. This makes their prose more startling to us, and to the other side at the time. It's easy to remember John C. Calhoun standing up and telling everyone that he views slavery as good in itself, not merely as a necessary expedient. Thinkers like Calhoun would say that slavery is good on a whole variety of fronts, so let's unpack them.

Slavery is good because it's good for black people. They were living in absolute savagery and barbarism in Africa, a degraded and depraved state of hopeless ignorance from which they had no hope of ever rising. Good white men took them in hand and have engaged for many centuries in a difficult process of uplifting them, for which black Americans should be grateful. They have been clothed, taught Christianity, and enjoy all the blessings of living in a modern, white state. Leave them on their own and their virtually infinite racial handicaps, by which they essentially mean that black people are a whole lot less intelligent than whites, and they must degenerate to squalor and starvation. You have to be staggeringly racist to buy into this, of course. Depressingly, most of the things argued under this heading remain staples of American discourse in only slightly modified form.

Slavery protects black people from free labor, which is evil. The wage laborer, Positive Good people would tell us, has no bonds of affection or familiarity with their boss. If they are injured or infirm, they are let go. They can be fired at a moment's notice should the market take a dip. Their lives are forever insecure. An enslaved person, despite being subject to horrific violence at whim, the destruction of families, and all the rest, is actually quite comfortable and well provided for. Elderly enslaved people have light or no duties. Enslavers provide medical care. The relationship is not one of a boss and an employee, but rather the warmer one of a kind of extended family. Not enough bowels have ever existed, nor sewers to serve them, to contain all this. Enslaved people who can't labor are at risk of being sold at a loss just to be rid of them and failing that providing for them generally falls to other enslaved people. Likewise the enslaved are fed, clothed, and housed not out of the enslaver's boundless goodwill but out of the labor stolen from their bodies. They build their cabins. They raise their food. They make their clothes. Or if they don't, they're provided for out of cash money that the enslaver extracted from them though the business end of a whip.

Slavery is good for white people. By placing the enslaved on a level no white man will ever experience, all white men are elevated and made, in a way, equal. This spares the South the troubling labor and class strife that threaten to set Yankees forever against one another. There are no classes in the South, because every white man is a member of the enslaver class by proxy and can expect at least on occasion to enjoy exercising the prerogatives of enslaving. Furthermore, it creates such wealth for whites that they might rise up and become disinterested statesmen who care only for the public good and virtue rather than the advancement of their own careers and endless currying of favor. You know that's true because those same disinterested statesmen are the ones telling it to you.

Everyone is doing it. Southerners like to play classicist. Some of them can carry it off, but peppering speeches with allusions to Antiquity and fragments of Latin is also just a done thing to show off. This makes them exactly as fun to read as you'd expect, particularly when they pick things almost at random. They are not shy about pointing out how Greece and Rome, those paragons of white civilization, were enslaving civilizations. They might tell you the same of Egypt, depending on how they feel about Egypt's whiteness. (A lot of effort goes into making Egyptians at least semiwhite in order to praise them and remove the obvious anomaly of an African people who constructed monuments that impress white people.) Slavery did not bring on the fall of those great polities, but instead contributed directly to their greatness. Furthermore, every great empire then current practiced slavery too, most of them not all that far away on colonies in the Caribbean. If slavery is so bad and so destructive, how did those empires ever rise to greatness? If Americans abandon it, they must surely surrender their potential to follow in that glory. All that is "proved" later on by the perceived failure of British emancipation in the West Indies, as well as by the poverty of Haiti. Those failures, incidentally, consist almost completely in that freedpeople don't want to devote their entire lives to enriching their former enslavers. Instead, for some reason that can't be fathomed most of them would rather adopt some kind of subsistence or safety first agriculture built around providing for themselves and their families, only then looking at growing cash crops for the world market.

I think it's quite clear from all of this that these are arguments not well supported by the facts and impossible to sustain without incredible levels of white supremacy, but white Americans have a nearly inexhaustible supply of the latter to grant the arguments plausibility and a surfeit of financial, social, and emotional investment in that white supremacy and the rewards it brings to overlook inconvenient details. Not everyone in the system gets paid the same, or in the same way, but the benefits of whiteness are spread well enough that it's rare for any sustained attack upon it to gain traction.

Recommended Reading

The study of white supremacy, proslavery argument, and racial theory are all deeply intertwined. The best starting point is probably Tise's Proslavery. Next steps get us into more complicated stuff. Winthrop Jordan's White Over Black (or the more reader-friendly version: The White Man's Burden) and George Frederickson's The Black Image in the White Mind are good overviews of the broader question. For a more history of science approach, Dain's A Hideous Monster of the Mind is very helpful, as is Painter's A History of White People.