The Mitchum v. Foster case in 1971 revolved around a Florida statute that prohibited the sale of obscene material and was used to close down Book Mart, owned by Robert Eugene Mitchum. The Supreme Court had to decide whether federal courts could intervene in ongoing state proceedings under certain circumstances, specifically regarding civil rights issues as per Section 1983 of the U.S. Code (a law allowing individuals to sue for civil rights violations). Previously, it was held that federal courts couldn't interfere due to an anti-injunction statute unless expressly authorized by Congress. In this case, however, the Supreme Court ruled in favor of Mitchum stating that Section 1983 did constitute an express exception from the anti-injunction act because it created a uniquely federal remedy against incursions under claimed authority of state law upon private constitutional rights. Therefore, if someone's constitutional rights were being violated according to this section they could seek help from Federal Courts even if there were ongoing State proceedings.

In the dissenting opinion for Mitchum v. Foster, Justice Rehnquist argued that federal courts should not have the authority to intervene in state court proceedings unless it is absolutely necessary. He contended that Congress did not intend for Section 1983 of Title 42 (Civil Rights Act) to be an exception to the anti-injunction statute when they enacted it, and therefore disagreed with the majority's interpretation of Congressional intent. He also expressed concern about potential overreach by federal courts into state matters, which could disrupt a delicate balance between state and federal judicial systems established by principles of comity and federalism. Furthermore, he warned against undermining public confidence in state judiciaries if their decisions are frequently second-guessed or overturned by lower-level federal courts.