The Guarantee Clause, also known as the Republican Form of Government Clause, is in Article IV, Section 4 of the United States Constitution, and requires the United States to guarantee every State a Republican form of government and provide protection from foreign invasion and domestic violence.

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.

Cases such as Luther v. Borden held the Guarantee Clause to be a political question to be resolved by Congress. Relying on that understanding, the Reconstruction era Congress disestablished ten state governments during peacetime and placed them under military rule. The law, known as the First Reconstruction Act, found those states to be unrepublican under the Guarantee Clause.[4][5] The Supreme Court acquiesced to the disestablishment in Georgia v. Stanton (1868).[6] Later, Congress also excluded elected legislators (a power recognized in Luther) when it faced "an election dispute created by state measures to suppress black voter turnout."[6]