In a single republic, all the power surrendered by the people, is submitted to the administration of a single government; and usurpations are guarded against by a division of the government into distinct and separate departments. In the compound republic of America, the power surrendered by the people, is first divided between two distinct governments, and then the portion allotted to each, subdivided among distinct and separate departments. Hence a double security arises on the rights of the people. The different governments will control each other; at the same time that each will be controlled by itself.
James Madison The Federalist Papers Federalist No. 51