As James Madison explains it in Federalist No. 43, it means that “In a confederacy founded on republican principles, and composed of republican members, the superintending government ought clearly to possess authority to defend the system against aristocratic or monarchial innovations.”
He goes on: “The more intimate the nature of such a Union may be, the greater interest have the members in the political institutions of each other; and the greater right to insist that the forms of government under which the compact was entered into, should be substantially maintained.”
Of course, there’s no real chance in the modern era that any state will become a “monarchy” or “aristocracy” in the 18th-century sense. So why does the Guarantee Clause matter, and what does it mean? How does one determine whether a state has maintained a “republican form of government?”
Ordinarily we would turn to the Supreme Court for an answer to a question of this sort. But here, the court has deferred to Congress. In Luther v. Borden in 1849 — a suit that concerned the authority of a Rhode Island government that still operated under its original royal charter and which rested on the Guarantee Clause — Chief Justice Roger Taney (later of Dred Scott infamy) declared that
Under this article of the Constitution, it rests with Congress to decide what government is the established one in a State. For as the United States guarantee to each State a republican government, Congress must necessarily decide what government is established in the State before it can determine whether it is republican or not.
Taney’s ruling held strong, a little more than 60 years later, in Pacific States Telephone and Telegraph Co. v. Oregon, when the court rebuffed a claim that the Guarantee Clause rendered direct referendums unconstitutional by stating that it was beyond the scope of the power of the Supreme Court to enforce the guarantee of a republican government. “That question,” wrote Chief Justice Edward White in his majority opinion, “has long since been determined by this court conformably to the practice of the government from the beginning to be political in character, and therefore not cognizable by the judicial power, but solely committed by the Constitution to the judgment of Congress.”
This remains the court’s view. But it’s not the only view. In his famous dissent in Plessy v. Ferguson in 1896, Justice John Marshall Harlan cited the Guarantee Clause in his brief against Louisiana’s Jim Crow segregation law. If allowed to stand, he wrote,
there would remain a power in the States, by sinister legislation, to interfere with the blessings of freedom; to regulate civil rights common to all citizens, upon the basis of race; and to place in a condition of legal inferiority a large body of American citizens, now constituting a part of the political community, called the people of the United States, for whom and by whom, through representatives, our government is administrated. Such a system is inconsistent with the guarantee given by the Constitution to each State of a republican form of government, and may be stricken down by congressional action, or by the courts in the discharge of their solemn duty to maintain the supreme law of the land, anything in the Constitution or laws of any State to the contrary notwithstanding.
In this vision of the Guarantee Clause, the touchstone for “a republican form of government” is political equality, and when a state imposes political inequality beyond a certain point, Congress or the federal courts step in to restore the balance.
In a 2010 article for the Stanford Law Review, Jacob M. Heller called this a “death by a thousand cuts” approach to enforcement, one where lawmakers and courts understand that “anything that impedes on the state’s republican form is one step closer to eventual unraveling of a state’s republican form of government.”
Even if wielded in this manner, the Supreme Court may determine there’s nothing the judiciary can do to insure a “republican form of government.” Chief Justice John Roberts suggested as much in Rucho v. Common Cause in 2019, with a note that the plaintiffs’ objection to an earlier legislative map of North Carolina seems “more properly grounded in the Guarantee Clause,” but that the court “has several times concluded” that the clause “does not provide the basis for a justiciable claim.”