The Real Constitution: Article IV, Section IV (Pledge of the United States to the Individual States)

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.

A lot of this section has its roots in Edmund Randolph's Virginia Plan; the Articles of Confederation did not explicitly guarantee "a Republican Form of Government" to the states, perhaps because it was really subordinate to the states and not the other way around. Randolph also worried that the confederation "produced no security against foreign invasion" and "could not check... a rebellion in any [state]".