Rep. Doug Lamborn (R-Colo.) questions a witness during a House Veterans Affairs subcommittee field hearing on VA hospitals and prescription drugs for veterans in the state Capitol in Denver on May 20, 2016. (David Zalubowski/AP)

Rep. Doug Lamborn (R-Colo.) is pursuing legal action to keep his name on the June 26 primary ballot after the Colorado Supreme Court ruled that hundreds of signatures on his candidate petitions were collected by people who did not live in his district.

“We recognize the gravity of this conclusion, but Colorado law does not permit us to conclude otherwise,” the court wrote in a decision released Monday afternoon. Lamborn’s campaign pledged immediately to fight the decision.

“We are disappointed by the outcome and believe it was wrongly decided,” said Lamborn spokesman Dan Bayens. “We are immediately bringing an action in federal court to overturn the part of Colorado law that deprives voters who have petitioned to have Congressman Lamborn on the ballot of their constitutional rights.”

Lamborn, first elected in 2006, has faced primary challengers in almost every campaign, holding off Republicans who usually challenged him from the center-right, saying he had been an ineffective representative for central Colorado. His first campaign for the 5th Congressional District, which is based in Colorado Springs and has never been won by a Democrat, was backed by the conservative Club for Growth.

This year, Lamborn faced four Republican challengers, including Darryl Glenn, the party’s unsuccessful 2016 nominee for U.S. Senate, and state Sen. Owen Hill, who had outraised the incumbent. Lamborn initially qualified for the ballot, with 1,269 valid signatures — more than the 1,000 required. But a group of Republican activists sued, pointing out that at least one of Lamborn’s petition gatherers had paid California taxes in 2017 and did not fully reside in Colorado.

If Lamborn is unsuccessful in court, Colorado law allows him to compete in the primary as a write-in candidate, a tactic some other politicians have used after stumbling over signature requirements. The path to regain ballot access after losing it is trickier. Last month, a Democrat seeking a competitive House seat in Iowa withdrew a ballot petition after her campaign manager admitted forging signatures; the state’s Democratic attorney general later ruled that she could not be nominated by any other means.

Colorado’s Democrats cheered today’s court decision, saying that Walker Stapleton, the front-runner for the Republican gubernatorial nomination, had also relied on out-of-state petitioners.

“Republicans like Walker Stapleton and Doug Lamborn have wasted hundreds of thousands of dollars bringing out-of-state fraudsters to Colorado to collect signatures that were eventually deemed worthless,” said Eric Walker, a spokesman for the party. “If you can’t be trusted to oversee a signature-gathering operation without committing fraud, why on earth should Coloradans trust you to hold elected office?”