But who is “they”?
This is the problem with counterfactual, “what if?” history—it has to take into account too many variables.
Had the United States as we know it not come together, and had there been two separate nations (slave and free soil) instead, we can’t really know how the historical developments that we now call “westward expansion” would have played out.
Would Spain still have retroceded Louisiana to the French, if they weren’t worried about dealing with a single, expansionist United States? If they did, would the French have sold it to the free soil American nation, or to the slave American nation, or to no-one at all?
Would either the free soil nation or the slave nation have been as expansionist as the United States was in the era of James Polk? If they were, would either one have been belligerent enough and powerful enough to prod Mexico into a war, and defeat Mexico in order to take over the lands that Mexico ceded to the US under the Treaty of Guadalupe-Hidalgo? What would have happened in the tug-of-war over the Oregon Territory? Would there still have been a “Fifty-four forty or fight” campaign, in either American nation?
And all this doesn’t even take into account other questions of international relations. How, for example, would the two new nations have interacted with Britain? Would the emergence of King Cotton as an economic force in the slave region have led to an alliance with Britain, which purchased cotton for its textile mills? And would that alliance have led to hostilities or war with the free soil nation, with Britain taking the side of the slave nation? It’s pretty certain that the War of 1812 would not have happened, or at least not as we understand it.
Remember, too, that the north-south split was not as pronounced in 1789 as it would become later. The census of 1790 counted over 20,000 slaves in New York, over 10,000 in New Jersey, almost 3,000 in Connecticut, and almost 4,000 in Pennsylvania. The only state with no slaves at all was Massachusetts (Maine, counted separately in the census, was part of Massachusetts).
The gradual abolition clauses in states like New York reflected revolutionary-era concerns about the sanctity of property, and however immoral we find the idea of property in human beings, this was a live and important issues for the people of the time. Jefferson, in the section entitled “Manners” in his Notes on the State of Virginia, expressed the hope that emancipation would come, and that it would come with the agreement of slaveowners themselves.
I teach United States history to college students, and while i understand the inclination to make clear-cut moral judgments about historical figures like the framers of the Constitution, i think it’s historically more productive to understand them and their decisions, rather than simply debating whether or not those decisions were “morally reprehensible.”
In a similar vein, i think that asking “Was Abraham Lincoln a racist?” is an unproductive historical question. In my classes, we deal with Lincoln’s attitudes to racial differences, and to free soil and slavery, and other questions related to his policies before and during the Civil War. Those are all important issues, and are worth understanding. But questions like “Was Lincoln a racist?” obscure far more than they illuminate, in my opinion.