Yale and the Price of Slavery
Presentism is very often advanced in defense of America's founders. It is comforting to think that their generation, so distant in time from us, lived in a condition of moral ignorance, and thus innocence, regarding slavery. But that is not the case. Even Thomas Jefferson, some of whose statements exhibit an almost demented racism, could see clearly that slavery utterly compromised the nation: "I tremble for my country when I reflect that God is just; that His justice cannot sleep forever," Jefferson wrote. "The Almighty has no attribute which can take side with us."

George Washington was an enthusiastic slaveholder in his early decades, buying slaves to build himself a plantation empire; but by the end of his life he found slavery repugnant. In his will Washington freed his slaves and specified that the children be educated, believing that with education and training the freed children of slaves could take a more fruitful and productive place in Virginia society. If we accept the statement that "it's downright inappropriate to render a moral judgment" on slavery, we are more willing to accept slavery than George Washington was.

If the founders had such misgivings over slavery, how is it that they allowed slavery to continue? The answer is not that they didn't know any better, but that they kept slavery so the Southern states would join the union. It was a transaction, a deal, just like the deal that put the national capital on the Potomac in exchange for the federal assumption of states' debts — and not unlike the deal the Hairstons made in causing their kin to disappear. With their eyes open, the founders traded away the rights of African-Americans, many of whom had fought bravely in the Revolution, so that the national enterprise could go forward.
http://www.nytimes.com/2001/08/18/opinion/18WIEN.html