However, slavery in America's formative years co-existed with an institution that is not well-understood today: indentured servitude. The existence of this form of employment helped salve the consciences of white slave-owners in the formative period, I think. It also, I think, confused the issue of slavery's injustice among non-slave-holding whites. So perhaps it would be helpful to remind people of what this institution was, and why it ended.
Passage to America wasn't cheap, and most people who wanted to come here from Europe had little means to pay for it. The Pilgrim Fathers of New England had become part of a colonizing scheme in which their economic production was supposed to repay and supply profit for the investors who put up the money to found the colony. The investment never paid off for the people who put up the money for it. Colonization was a money sink.
Eventually, the promoters of colonies found a way to supply manpower (and citizens) for their colonies through the use of indentured servitude. This was a contract for labor in which a person would agree to essentially work for free for a term of years in order to pay the costs of one's transport to America. People in labor-hungry America looking for servants, farm-workers, and so on would buy these contracts when the indentured servants arrived.
Indentured servitude was very like another form of contract labor from the period, the apprenticeship. An apprentice was "articled" to a master craftsman for a term of years. The apprentice moved into the craftsman's establishment and became part of his household. The articles of apprenticeship required the 'prentice to be fed and clothed and so on while he was being taught his trade. At the end of his term, he became a journeyman and was released to seek his own employment.
An indentured servant wasn't being taught a trade, usually, though domestic servants were considered part of the extended household. On the other hand, if you were a farm laborer, your servitude might be little better than that of a slave; this was especially true if you were a rebellious Irishman or Scot set to work on a cane plantation in the West Indies. In the normal course of things, though, you were given a small amount of money at the end of your term and freed to do as you liked with yourself.
An enormous number of immigrants to America (not just the Thirteen Colonies) came as indentured servants, and they helped fill out the population of the expanding society. Some came unwillingly, as when many Scots were cleared out of the Highlands following Culloden and other social upheavals. Others came very willingly, particularly the Scotch-Irish. So many domestic servants were Irish (Scotch-Irish, from Ulster, not "Irish" Irish from the rest of the island), that the "Irish" maidservant became a stereotype.
Those who traded in black slaves but had tender consciences about it could say that slavery was really just the same kind of thing as indentured servitude. After all, many servants were treated kindly by their masters and were legally part of their household (of course, slaves weren't freed after their term, except when that term was their natural lives -- death was the only guaranteed end of their slavery). In the colonial period, too, manumission of slaves was easier and more frequently practiced, and in many places slaves could buy their freedom. Many slaves also worked for wages and were allowed to keep them. Free blacks lived even in the South, and some owned slaves themselves. So the situation was more fluid than it later became. And I think that the rhetoric over slavery (which was very gingerly not mentioned by name in the Constitution) was made more palatable by confusion with indentured servitude.
Indentured servitude eventually went out of fashion. The population had filled up, and labor was not so desperately needed. Domestic servants were still common, but they were now paid wages. Europeans looking to immigrate found other means of paying for their passage; in many cases, earlier immigrants helped bring over family members left behind. Apprenticeships were still common, though eventually live-in arrangements faded out; today, apprenticeships are basically an educational enterprise like an extended internship or work-study. That left slavery by itself, in all its increasing ugliness.
As slavery became virtually the only form of involuntary servitude left (other than prisoners' work gangs), so the ideology of race became ever more nasty. A low opinion of black people was necessary to justify their enslavement. At the same time, a lot of sentimental bilge about how their masters cared for their slaves -- emotionally, as well as physically -- was probably still due to a confusion over the nature of their work relationship. Slaves with whom one interacted on a daily basis could be seen as the former indentured servants were, as laborers who "belonged," not merely who were "owned." This self-delusion helped the slave-owning classes justify their participation in chattel slavery: what you don't want to see, you train yourself not to see.
Fear of slave revolts hardened Southerners' attitudes toward slaves. The presence of free black persons competing for wages next to enslaved black persons challenged the status assigned to slaves, so it was made harder for free black persons to operate in slave States. Manumission became harder to legally accomplish, even for willing owners, since unwilling owners didn't want the example given for other masters to follow or for other slaves to desire. And the cotton gin finally made plantation slavery profitable. Slavery not only stood out as a "peculiar institution," unlike other forms of labor, but all social mobility was excluded from it, reducing the enslaved to the level of domestic livestock. And as the slaves were further degraded, so the ideology of race became ever more fixed and fanatical.
Today, some descendants of Irish, Scotch-Irish, or Scottish extraction like to say that their ancestors were brought to this country as "slaves," but this is inaccurate. No doubt their ancestors were often looked down on socially, but they were not enslaved, and appealing to their "unfree" status is a form of virtue-signaling, hitching a ride on the moral superiority that today's progressives assign to victims. In some cases, it's the same kind of rhetorical confusion played out three hundred years ago, but in reverse: instead of hinting that enslaved blacks suffered no worse than indentured whites, it now says that indentured whites were as oppressed as enslaved blacks.