In the 1970 case of Samuels et al. v. Mackell, District Attorney of Queens County, et al., the U.S Supreme Court ruled on whether a federal court could intervene in ongoing state criminal proceedings under certain circumstances. The plaintiffs were individuals charged with violating New York's anti-loitering law and sought to have it declared unconstitutional by a federal district court before their trials began in state courts. They argued that they should not be required to wait until after conviction to challenge the constitutionality of the statute under which they were being prosecuted. The Supreme Court held that when there is an ongoing state prosecution, federal courts should generally refrain from granting declaratory relief unless there are exceptional circumstances where necessary to prevent immediate irreparable injury. The Court reasoned that allowing such intervention would disrupt the balance between state and federal judicial responsibilities and potentially interfere with pending state proceedings.

In the dissenting opinion for Samuels et al. v. Mackell, District Attorney of Queens County, et al., Justice Harlan argued that the majority's decision to deny federal courts jurisdiction over state criminal proceedings was a misinterpretation of Younger v. Harris and other related cases. He believed that this ruling would unnecessarily restrict access to federal relief in situations where it might be warranted, such as when there is an ongoing violation of constitutional rights or when irreparable harm could occur before a state court has had time to act on the matter at hand. Furthermore, he expressed concern about potential abuses by local prosecutors who may use their power to harass individuals with repeated prosecutions without any intention of obtaining convictions - something which he felt should be preventable through intervention by federal courts if necessary.