In the case of Oregon v. Mitchell in 1970, the U.S. Supreme Court was asked to rule on whether Congress had overstepped its authority by passing legislation that lowered the voting age from 21 to 18 for all elections—federal, state and local—and banned literacy tests as a prerequisite for voting in state and local elections. The court ruled separately on each issue: it upheld Congress's power to lower the voting age for federal elections but not for state or local ones; it also affirmed Congress's right to ban literacy tests nationwide under Section Five of Voting Rights Act of 1965 which gave them enforcement powers regarding racial discrimination in voting laws. This decision led directly to ratification of the Twenty-Sixth Amendment, which standardized minimum voting age at eighteen across all types of election.

In the case of Oregon v. Mitchell, 1970, Justice Harlan dissented on two main points. Firstly, he disagreed with the majority's decision to uphold Congress' power to set a minimum voting age for state and local elections under the Equal Protection Clause of the Fourteenth Amendment. He argued that this was an overreach of federal authority into areas traditionally controlled by states - in this case, determining voter qualifications. Secondly, Harlan also expressed disagreement with upholding Section 4(e) which prohibited English literacy tests as a prerequisite for voting in jurisdictions where more than five percent of citizens were American Indian, Asian American or Spanish-speaking unless they had completed sixth grade in an accredited school where instruction was predominantly conducted in English language. He believed it violated principles of equal protection because it discriminated against other non-English speaking groups not included within its scope.