Republican congressional candidate Brad Dempsey is suing Secretary of State Jena Griswold for a spot in Colorado’s 7th Congressional District primary after her office determined his campaign didn’t submit a sufficient number of valid petition signatures.

In a lawsuit filed late Wednesday in Denver District Court, Dempsey and four unaffiliated Jefferson County voters say that unaffiliated voters should be able to sign nominating petitions for major party candidates in Colorado because state law lets them vote in primary elections.

Citing Proposition 108, a 2016 ballot measure approved in 2016 that permits unaffiliated voters to cast ballots in either the Democratic or Republican primary, Dempsey said in a statement that Colorado voters made it clear that they wanted to enable unaffiliated voters to have a say on which candidates would receive the nomination from the respective major political parties in Colorado.

He added that he filed the lawsuit “to make sure that the First Amendment rights of thousands of voters who signed my petitions to nominate me to the primary ballot are fully honored.”

If Dempsey and his co-plaintiffs succeed, he’ll join Republicans Erik Aadland, Tim Reichert and Laurel Imer on the June 28 primary ballot for the seat held by retiring U.S. Rep. Ed Perlmutter, the Arvada Democrat who has represented the district for eight terms. Another Republican, Carl Andersen, is also suing to be included in the primary, claiming that hundreds of signatures on his petitions were wrongly rejected.

Democrat Brittany Pettersen, a Lakewood state senator, is her party’s presumptive nominee in the competitive district, which covers Jefferson County, Broomfield and six mountain counties.

Under current practice, the Colorado Secretary of State’s Office only considers petition signatures valid if they belong to voters registered with the same party as the candidate. Under those requirements, Dempsey fell 251 signatures short of the 1,500 required for major party congressional candidates to make the ballot. His lawsuit argues that 287 of the 983 signatures Griswold’s office rejected should count because they belong to unaffiliated voters who otherwise meet the requirements to sign a petition.

“In rejecting otherwise valid petition signatures from unaffiliated voters, the Secretary of State’s Office has applied old law that has been replaced by Proposition 108 and deprived voters of their right to nominate their Republican candidate of choice to the primary ballot,” Dempsey said.

His position puts Dempsey at odds with a group of Colorado Republicans who recently attempted to repeal Proposition 108, arguing that the state GOP should be able to choose its own nominees without the participation of voters who don’t belong to the party. A federal judge on April 8 dismissed a request from the group to issue a preliminary injunction, which would have prevented unaffiliated voters from voting in this year’s primary.

Dempsey’s lawsuit points out that Griswold and other statewide officials argued in favor of maintaining Proposition 108 in that case.

“Moreover,” Dempsey’s lawsuit says, “the Secretary should not be heard advocating for the inclusion of Unaffiliated voters in Colorado’s primary system in recent federal case law, and then subsequently taking conflicting action to reject signatures from and exclude the very same Unaffiliated voters who have taken the initiative to sign the Petition so they can have the option to vote for Dempsey in the primary election.”

A spokeswoman for Griswold, a Democrat, declined to comment on Dempsey’s lawsuit, citing a policy against weighing in on pending litigation.

The case is set for a hearing Tuesday afternoon before Judge J. Eric Elliff.

Griswold faces an April 29 deadline to certify the primary ballot.