Constitution of the United States of America
Article IV: Interstate Relations
Article 4, Section 4:  Guarantee of Republicanism

4.4.1
 The United States shall guarantee to every State in this Union a Republican
Form of Government,

4.4.2  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.
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