In win for Colorado GOP, judge rules that state makes it too hard to close primaries to unaffiliated voters

Seth Klamann, The Denver Post on Apr 1, 2026

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DENVER — A federal judge has struck down a key part of a 2016 ballot initiative that sets a high threshold for Colorado political parties to close their primary elections to unaffiliated voters, delivering a significant win to conservatives within the state Republican Party.

The ruling did not fully strike down Proposition 108, which opened up primaries to participation by party faithful and unaffiliated voters alike. But in the decision issued on Tuesday, U.S. District Court Judge Philip A. Brimmer wrote that the law puts a “severe burden” on parties that want to opt out of the open primaries.

He ruled that part of the law violates the First Amendment’s protections of the freedom of association.

The law says that if a political party wants to close its primary so that only its own registered voters can participate, three-quarters of that party’s central committee must vote in support of the change. That threshold is unconstitutionally high, Brimmer wrote.

“The background and origin of the three-fourths opt-out provision is unclear,” Brimmer, who is based in Denver, wrote. “What is clear is that it constitutes an unusual and difficult barrier for the central committee to overcome, more akin to a hurdle to amend a foundational governing document, such as the United States Constitution, than a traditional means of regulating political parties.”

He noted that unaffiliated voters’ share of the Colorado electorate has grown, and they now make up a majority of registered voters in the state.

Brimmer’s ruling does not set a new threshold for parties to hit, should they want to close their primaries, and his order does not give specific direction for how the state or parties should proceed. The state GOP meets April 11 for its state assembly, where it will nominate some candidates for this year’s June 30 primaries.

The state GOP filed the lawsuit against the Colorado Secretary of State’s Office in 2023 under the leadership of conservative hardliner Dave Williams. He had blasted Prop 108 as a “radical leftwing” effort and argued that closing the primaries was vital to protecting true Republican candidates.

Brimmer previously rejected the GOP’s request to immediately close primaries to unaffiliated voters in 2024, and the lawsuit dragged on for another two years — outlasting Williams and nearly outliving his successor, current GOP chairwoman Brita Horn. Horn is stepping down as party chair later this month.

The new ruling would apply to both the Republican and Democratic parties, should Democrats later choose to close their primaries, too.

Representatives for the two parties did not immediately return messages seeking comment Wednesday morning. Secretary of State Jena Griswold’s office was also not immediately able to comment.

In the decade since Prop 108 was passed, an ever-increasing share of voters has registered as unaffiliated, and no party has opted out of Colorado’s semi-open primaries under the ballot measure’s provisions.

But Republicans have tried at least three times, according to Brimmer’s ruling. In September 2023, more than 64% of the party’s central committee who voted on the motion supported closing the primaries, falling well short of the threshold.

Brimmer wrote that roughly 100,000 unaffiliated voters participated in Republican primaries in 2018. By 2022, that total had more than doubled to 231,000, accounting for 37.1% of ballots cast in those contests that year. In some counties, the unaffiliated voters rivaled the number of Republican voters participating in the GOP’s primaries in 2022 and 2024.

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