Abstract: This Article advances a novel theory of the political question doctrine by locating its foundations in a conundrum about ultra vires action, exemplified by the ancient question: Who will guard the guardians? The political question doctrine marks some questions as ultra vires the judicial power, or beyond the jurisdiction of courts to resolve. Correspondingly, designation of a question as political typically identifies it as lying within the jurisdiction of a nonjudicial institution to settle. Even after denominating a question as political, however, courts retain a responsibility to check actions by other institutions that overreach those institutions’ authority and thus are themselves ultra vires. The need for the judiciary to press to the outer limits of its jurisdiction to rein in ultra vires action by other institutions renders political question rulings less categorical, and also less distinct from merits decisions, than both judges and commentators have often imagined. The inescapable role of the courts in identifying ultra vires action by other branches also highlights the possibility of ultra vires action by the courts themselves. The paired risks of ultra vires action by the courts and ultra vires action by other branches if the courts could not assert jurisdiction to restrain them—both made vivid by the political question doctrine—define what this Article calls the ultra vires conundrum. The ultra vires conundrum, in turn, gives rise to what we might think of as ultimate political questions: What happens if courts err in their determination of the outer bounds of their own power? If the courts act ultra vires, do their decisions bind conscientious officials of other branches? And if not, who gets to decide when judicial action is ultra vires? Besides formulating the ultra vires conundrum and answering the questions that define its core, this Article solves a number of more traditional, interrelated puzzles about the political question doctrine that appear in a new light once the ultra vires conundrum lies exposed. It also traces previously unexplored connections between political questions and the ideal of the rule of law.