WASHINGTON — A federal appeals court Wednesday declared unconstitutional a Kansas law that required proof of U.S. citizenship to register to vote, with a rebuke for the Republican former secretary of state who had championed the requirement.

"This is a huge win for voters," said Rick Hasen, an election law expert at the University of California, Irvine. "It clears away a law that disenfranchised thousands but prevented no appreciable amount of voter fraud."

The law, which took effect in 2013 but was later blocked by a federal trial judge, was promoted by former state Secretary of State Kris Kobach, who for years said noncitizens were voting illegally. President Donald Trump appointed him to help lead a commission studying voter fraud, which was disbanded in 2018.

Dale Ho of the American Civil Liberties Union, which fought the law in court, said it was time for the state "to turn the page on Kris Kobach's sorry legacy of voter suppression, drop any further appeals, and work with us collaboratively in the interests of all Kansas voters."

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Under the law, residents were required to present a document, such as a passport or a birth or naturalization certificate, to prove their citizenship before they could register to vote. But a panel of the 10th U.S. Circuit Court of Appeals said Kobach failed to prove that the additional burden on voters was justified by actual evidence of fraud.

During the time the law was in effect, more than 31,000 applicants were prevented from registering. But the appeals court noted that over the past 19 years, at most 39 noncitizens managed to register, which the court called "incredibly slight evidence."

Kobach "failed to show that a substantial number of non-citizens successfully registered to vote," it said.

The ruling applies to the six states of the 10th Circuit — Colorado, New Mexico, Oklahoma, Utah and Wyoming, in addition to Kansas.

Kansas could appeal the decision to the full 10th Circuit court or to the Supreme Court. Kobach lost his bid for governor in 2018, and the current secretary of state, Scott Schwab, became the defendant. Representatives of Schwab and the state's attorney general, Derek Schmidt, said they were reviewing the ruling.