In the case of Rodriguez et al. v. Popular Democratic Party et al., 1981, the U.S. Supreme Court ruled on a dispute involving Puerto Rico's electoral system and its constitutionality under the Equal Protection Clause of the Fourteenth Amendment. The controversy arose when an interim appointment was made to fill a vacant seat in Puerto Rico's legislature without holding a special election, as per local law allowing political parties to appoint replacements if one of their members vacates their position before term completion. The plaintiffs argued that this practice violated their right to vote by not providing them with an opportunity to participate in choosing their representative directly through elections. However, the Supreme Court upheld Puerto Rico’s method for filling legislative vacancies mid-term by party appointments rather than special elections, stating it did not violate equal protection principles because there is no absolute constitutional requirement for immediate popular election replacement after every vacancy occurring within a term.

In the dissenting opinion for Rodriguez et al. v. Popular Democratic Party et al., Justice Brennan, joined by Justices Marshall and Blackmun, argued that the majority's decision was inconsistent with previous rulings on voting rights issues. They contended that the Court had previously held in Reynolds v Sims (1964) that any state legislation which significantly impacts an individual’s right to vote must be closely scrutinized to ensure it does not violate equal protection principles under Fourteenth Amendment of U.S Constitution. The dissenters believed this case should have been treated similarly as it involved a significant restriction on voters' ability to choose their representatives through interim appointments rather than elections following vacancies in Puerto Rico's legislature. They also disagreed with the majority's view about Puerto Rican citizens having less constitutional protections compared to those living within states, arguing instead they are entitled full protection under U.S constitution including fundamental right of suffrage.