In a 7-2 decision, the Supreme Court struck down an Arizona law Monday that requires people to submit proof of citizenship when they register to vote in a federal election.

Proposition 200 in Arizona established that anyone who wanted to vote in a federal election had to bring with them proof of citizenship. However, Justice Antonin Scalia noted that the state law conflicts with the 1993 Motor Voter Act federal law which requires states to "accept and use" the federal voter registration form, reports NBC.

Conservative justices Clarence Thomas and Samuel Alito dissented. Justice Thomas stated that the Constitution gave states the power "to determine the qualifications of voters in federal elections, which necessarily includes the related power to determine whether those qualifications are satisfied" in a long dissent, reports the New York Times.

According state officials, the law was adopted in 2005 to reduce voter fraud. However, civil rights groups said the law discouraged voting by legal immigrants and perpertrated voter suppression.

"Today's decision sends a strong message that states cannot block their citizens from registering to vote by superimposing burdensome paperwork requirements on top of federal law," said Nina Perales, vice president of litigation for the Mexican-American Legal Defense and Education Fund.

Citizenship is a requirement to vote in any federal election, and the federal registration form requires people to state, under penalty of perjury, that they are American citizens. States can use their own forms, but they must be equivalent to the federal form.

The Arizona Prop 200 went further than the federal form by requiring applicants to provide proof of citizenship. Arizona has used the law to reject 30,000 voter applications, according to the Brennan Center for Justice.

The Supreme Court ruling pointed out that Arizona can ask the federal government to include state-specific instructions on the federal form, and go to court if the government says no.