"The power the Constitution grants it also restrains. And though Congress has great authority to design laws to fit its own conception of sound national policy, it cannot deny the liberty protected by the Due Process Clause of the Fifth Amendment."
This dovetails nicely with the error in yesterday's ruling. In yesterday's ruling Congress was exercising its "great authority to design laws to fit its own conception of sound national policy" in an effort to protect the liberty outlined in the Fifteenth Amendment.
It was not, in some comparable way, denying the liberties of the states guaranteed by, say, the Tenth Amendment because no one denies that they have the authority to pass legislation to manage states' rights insofar as they pertain to equal access to voting. That was the whole point of the Fifteenth Amendment, to add that qualifier to the Tenth.
So the court was wrong on the Voting Rights Act because it denied Congress its constitutional authority without demonstrating what it demonstrated with the Defense of Marriage Act: that Congressional action "denied the liberty protected by" the Constitution.