Warren Petersen Backs Trump DOJ in Supreme Court Voter Registration Fight

Arizona Senate President Warren Petersen (R-AZ), a current candidate for state attorney general, expressed enthusiasm this week after the Trump administration filed a brief urging the Supreme Court to take up a legal dispute over Arizona's voter registration laws.

"Thrilled the Trump DOJ is backing Arizona's proof-of-citizenship requirements," Petersen wrote on X. "Only U.S. citizens should vote in our elections. Excited for SCOTUS to take up this important case."

The brief, filed by Acting Solicitor General Hashim Mooppan in May 2026, asks the Supreme Court to grant review of a petition brought by the Republican National Committee challenging two Ninth Circuit rulings.

The first found that Arizona's program to remove noncitizens from voter rolls within 90 days of a federal election was blocked by the National Voter Registration Act.

The second found that Arizona's requirement that residents provide documentary proof of citizenship, such as a passport, driver's license, or birth certificate, when registering to vote using the state form was also unlawful under the same federal law.

DOJ's Argument

However, the Department of Justice brief argues the Ninth Circuit got both questions wrong.

On the voter roll issue, it contends that federal protections covering registered voters simply do not extend to individuals who were never eligible to register in the first place, namely, noncitizens.

Regarding the proof-of-citizenship requirement, the brief cites a 2013 Supreme Court decision, Arizona v. Inter Tribal Council of Arizona, which the administration says explicitly recognized that states may require information on their own registration forms that the federal form does not.

The Ninth Circuit's rulings had invalidated the measures, which Arizona enacted in 2022 following the 2020 election. Furthermore, critics argued they would suppress legitimate voter participation.

The Supreme Court had previously granted partial emergency stays of the lower court rulings ahead of the 2024 election, a signal the administration now cites as reason for the Court to hear the case in full.

Petersen, whose attorney general campaign has centered heavily on election integrity issues, has been a vocal backer of the laws throughout the litigation.