Dunn v. Blumstein,
405 U.S. 330 (1972)
Tennessee closes its registration books 30 days before an election, but requires residence in the State for one year and in the county for three months as prerequisites for registration to vote. Appellee challenged the constitutionality of the durational residence requirements, and a three-judge District Court held them unconstitutional on the grounds that they impermissibly interfered with the right to vote and created a "suspect" classification penalizing some Tennessee residents because of recent interstate movement. Tennessee asserts that the requirements are needed to insure the purity of the ballot box and to have knowledgeable voters.
Held: The durational residence requirements are violative of the Equal Protection Clause of the Fourteenth Amendment, as they are not necessary to further a compelling state interest. ... A period of 30 days appears to be ample to complete whatever administrative tasks are needed to prevent fraud and insure the purity of the ballot box.