Article IV, Section 4:
The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.
In Luther v. Borden, 1 the Supreme Court established the doctrine that questions arising under this section are political, not judicial, in character and that
it rests with Congress to decide what government is the established one in a State . . . as well as its republican character. 2 Texas v. White 3 held that the action of the President in setting up provisional governments at the conclusion of the war was justified, if at all, only as an exercise of his powers as Commander-in-Chief and that such governments were to be regarded merely as provisional regimes to perform the functions of government pending action by Congress. On the ground that the issues were not justiciable, the Court in the early part of this century refused to pass on a number of challenges to state governmental reforms and thus made the clause in effect noncognizable by the courts in any matter, 4 a status from which the Court's opinion in Baker v. Carr, 5 despite its substantial curbing of the political question doctrine, did not release it. 6
Similarly, in Luther v. Borden, 7 the Court indicated that it rested with Congress to determine the means proper to fulfill the guarantee of protection to the states against domestic violence. Chief Justice Taney declared that Congress might have placed it in the power of a court to decide when the contingency had happened that required the Federal Government to interfere, but that instead Congress had by the act of February 28, 1795, 8 authorized the President to call out the militia in case of insurrection against the government of any state. It followed, said Taney, that the President
must, of necessity, decide which is the government, and which party is unlawfully arrayed against it, before he can perform the duty imposed upon him by the act of Congress, 9 which determination was not subject to review by the courts.
In recent years, the authority of the United States to use troops and other forces in the states has not generally been derived from this clause and it has been of little importance.