The United States guarantees that every State government will be the Republican Form of Government. This guarantee is the “Guarantee Clause” in the Constitution, Article IV, Section 4. 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.”
The first clause is called the “Guarantee Clause”.