Article Four of the United States Constitution
This clause, sometimes referred to as the Guarantee Clause, has long been at the fore-front of the debate about the rights of citizens compared to those of the government. The Guarantee Clause mandates that all U.S. states must be grounded in republican principles, such as the consent of the governed.  By ensuring that all states must have the same type of government (a republic), the Guarantee Clause is one of several portions of the Constitution which mandates symmetric federalism between the states.