Unless the plaintiff has a personal stake in the outcome, Article III of the United States Constitution requires federal courts to dismiss a plaintiff’s claim for lack of standing. That much is clearly established by decades of precedent. Less understood, however, is the degree to which Article III also requires defendants to possess a personal stake. The significance of defendant standing often goes unnoticed in case law and scholarship, because the standing of the defendant in most lawsuits is readily apparent: any defendant against whom the plaintiff seeks a remedy has a personal interest in defending against the plaintiff’s claim.

But the issue of standing to defend takes on outsized importance when third parties who are not targeted by the plaintiff’s requested remedy seek leave to intervene in order to oppose the plaintiff’s claim for relief. In cases featuring intervenor-defendants—often cases that concern important issues of public law—the personal-stake requirement becomes a real and not merely theoretical concern for the defendant. The problem is well illustrated by pending cases that address the constitutionality of California’s Proposition 8 and the federal Defense of Marriage Act. In each case, the executive branch officials named as defendants declined to defend the challenged law, prompting a nonparty with a questionable personal stake to seek to intervene to defend against a plaintiff’s claim. The prevailing plaintiff-centered model of standing does not lend itself readily to assessing whether such volunteer defendants have an interest sufficient to create a case or controversy.

This Article develops a model of defendant standing based on the functions that standing doctrine is intended to serve, and derived from the cases in which the U.S. Supreme Court has considered the personal stake of defendants under Article III. Under this model, absent a traditional injury in fact, intervenor standing to defend in public law litigation is appropriate only where state or federal law confers on the intervenor the authority to represent the government’s interest. This Article then illustrates the application of that model in the Proposition 8 and DOMA cases, and concludes that the intervenors in the Proposition 8 litigation do have standing to defend, while the intervenors in the DOMA litigation do not.