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.
ArtIV.S4.1 Guarantee Clause
ArtIV.S4.1.1 Guarantee of a Republican Form of Government
ArtIV.S4.1.1.1 Guarantee of a Republican Form of Government: Historical Background
ArtIV.S4.1.1.2 Guarantee of a Republican Form of Government: Doctrine and Practice