A coalition of non-profit groups have filed a lawsuit alleging the Arizona Secretary of State's Office is unconstitutionally blocking tens of thousands of people from voting with its registration rules.

The lawsuit, filed on behalf of the League of United Latin American Citizens Arizona and Arizona Students’ Association, focuses on the state's practice of treating federal and state voter-registration forms differently.

The biggest difference is the level of documentation needed to establish U.S. citizenship.

Under federal law, voters need only to sign a form swearing, under penalty of perjury, that they are a citizen. Arizona law, however, requires voters to provide documents proving their citizenship, such as a driver's license or birth certificate.

The disparity led Arizona to create a two-track voting system in which voters who only use the federal form only get a federal ballot. People who register using Arizona's voter form get the full ballot, from local elections to president.

Plaintiffs in the lawsuit allege Arizona Secretary of State Michele Reagan's office is implementing the state's proof of citizenship requirement unconstitutionally.

According to the suit, Reagan instructs election officials to reject any state voter-registration forms that don't include documented proof of citizenship. However, it states, thousands of those voters are still eligible to vote in federal elections but aren't being registered for either.

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About 26,000 voters in Maricopa County alone have been disenfranchised, according to the Campaign Legal Center, a non-partisan group supporting the plaintiffs.

"Arizona’s policies place unnecessary and irrational burdens on the right to vote and fly in the face of the Constitution’s promise of equal protection," Danielle Lang, an attorney for the center, said in a statement.

The plaintiffs also allege Arizona's proof of citizenship requirement makes it difficult to register new voters through voter drives, discouraging young people from signing up.

However, Reagan defended the state's dual voter-registration system in a statement. She noted Arizona voters approved the proof of citizenship requirement more than a decade ago, through a 2004 ballot initiative.

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"While it’s puzzling to understand why this lawsuit is being filed now, I think voters still want the state to verify eligibility to ensure election integrity and discourage fraud," Reagan wrote.

Arizona began treating state and federal voter registration differently after the U.S. Supreme Court ruled in 2013 that it cannot impose its registration requirements on federal elections.

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