The Constitution provides, and all the States have accepted the provision, that "the United States shall guarantee to every State in this Union a republican form of government." But if a State may lawfully go out of the Union, having done so it may also discard the republican form of government; so that to prevent its going out is an indispensable means to the end of maintaining the guaranty mentioned; and when an end is lawful and obligatory the indispensable means to it are also lawful and obligatory.

Fourth of July Address to Congress (1861)