"Our acceptance of race-based state action has been rare for a reason. “Distinctions between citizens solely because of their ancestry are by their very nature odious to a free people whose institutions are founded upon the doctrine of equality.” Rice v. Cayetano, 528 U. S. 495, 517 (2000) (quoting Hirabayashi v. United States, 320 U. S. 81, 100 (1943)). That principle cannot be overridden except in the most extraordinary case."