In Callanan v. United States, the Supreme Court ruled on whether a conviction under federal law for conspiracy to obstruct commerce by extortion was valid when there was no evidence of an overt act in furtherance of the conspiracy. The petitioner, Martin J. Callanan, argued that his conviction should be overturned because it lacked proof of such an act. However, the court held that under the Hobbs Act (which makes it a crime to obstruct interstate commerce by robbery or extortion), proving an overt act is not necessary for a conspiracy charge and upheld Callanan's conviction. The majority opinion stated that while some statutes require proof of an overt action to convict someone for conspiring to commit a crime, this requirement does not apply universally and depends on how each specific statute defines "conspiracy". In this case, they found nothing in the language or history of the Hobbs Act suggesting Congress intended to include such requirement. This decision affirmed that individuals could be convicted for merely planning criminal activities affecting interstate commerce even if those plans were never acted upon.

In the dissenting opinion for Callanan v. United States, Justice Brennan argued that the majority's interpretation of the federal conspiracy statute was overly broad and inconsistent with its legislative history. He contended that Congress intended to punish only those who actively participate in a criminal enterprise, not mere passive investors or associates. Furthermore, he disagreed with the majority's assertion that an agreement between two parties is sufficient to establish a conspiracy under this law; instead, he believed there must be evidence of some overt act in furtherance of their shared criminal objective. Finally, Justice Brennan expressed concern about potential abuses of prosecutorial discretion resulting from such a wide-ranging interpretation of conspiracy laws and warned against undermining fundamental principles of individual guilt and responsibility.