Still, the essentially permanent presence of some 11 million people whose labor and cultural contributions benefit the United States — yet who must exist on the margins of society — creates dilemmas both practical and moral.

And the issue has this in common with slavery: It’s a long-standing debate over fundamental rights that the nation’s democratic institutions have proven incapable of resolving, leading to increasingly bitter partisan conflict.

The ideological concept behind Obama’s grand slicing of the Gordian knot of immigration, if he attempts it, would be akin to that which drove Lincoln’s action: the president as liberator.