A federal judge struck down Kansas' sweeping proof of citizenship voter registration law on Monday, championed by Secretary of State Kris Kobach, an immigration hardliner who is running for governor and led President Trump's now-disbanded voter fraud commission.

Why it matters: This is a rebuke of Kobach, who helped fuel Trump’s unsubstantiated claims that millions of fraudulent votes had cost him the popular vote in 2016. Kobach, a Republican, claimed that the Kansas policy was intended to curb voter fraud, but his commission failed to provide concrete evidence of it doing so. The court said the law is unconstitutional and violates the National Voter Registration Act.

The details: The ruling comes exactly two months after U.S. District Judge Julie Robinson ordered that Kobach be held in contempt of court for refusing to abide by her previous temporary injunction, which ruled that the state must register voters blocked by the law.

In her scathing 100-plus page ruling, Robinson said: “[E]vidence leads the Court to the conclusion that tens of thousands of eligible citizens were blocked from registration before this Court’s preliminary injunction, and that the process of completing the registration process was burdensome for them."

Kobach's office said in a statement it plans to appeal the ruling.

The backdrop: Under the 2013 law, people registering to vote at the Department of Motor Vehicles are required to present documentary proof of citizenship, such as a birth certificate, naturalization papers, or a passport. Only three other states — Arizona, Georgia, and Alabama — have similar proof of citizenship laws. However, only Arizona implements it.