“The fundamental idea is that through the separation of powers and checks and balances, different voices—those of the President, the Senate, and the House of Representatives—can be expected to contribute to public debate about the ends and means of national policy. The notions are familiar: the President speaks as the nationally elected voice of the people generally; the Senate represents the states; and the House represents particular constituencies that often have highly local concerns. More generally, the President speaks for the nation, and members of Congress—while being concerned with matters of national import—speak especially for different constituent parts of the nation. This constitutional structure guarantees that diverse perspectives will contribute to dialogue about public policy.”