In a four-page order, Chief U.S. District Judge Philip A. Brimmer sent the lawsuit back to a state court in Denver County, where it was filed last week by a group of six voters.
Brimmer, who was nominated by former President George W. Bush, said Trump had not properly served Colorado Secretary of State Jena Griswold, a Democrat, or obtained her consent for removal as required by law.
"Because Secretary Griswold accepted service before Mr. Trump removed the case and she did not join in or consent to removal, the Court finds that removal was defective," Brimmer wrote.
A lawyer for Trump did not immediately respond to a request for comment Tuesday night.
In a statement, Sean Grimsley, an attorney representing the voters, said: "We are pleased with Chief Judge Brimmer’s decision rejecting former President Trump’s attempt to move this case to federal court. We look forward to presenting our clients’ case in state court.”
The watchdog group Citizens for Responsibility and Ethics in Washington and several firms, including Grimsley's, filed the lawsuit on behalf of the six voters.
Shortly before Brimmer issued his order, lawyers for Trump said in a filing that Trump "withdraws" his removal notice and "does not object to a remand to state court," while also arguing that Trump did not need Griswold's consent in seeking to move the case. Brimmer disputed that argument in his order.
A spokesperson for Griswold’s office did not immediately respond to a request for comment. The Colorado attorney general’s office declined to comment on active litigation.
Brimmer issued his decision days after Trump asked to have the lawsuit moved from state court to federal court because it cited the U.S. Constitution's 14th Amendment as a core argument.
The 14th Amendment says in part that no person shall hold any office who has “engaged in insurrection or rebellion” after having taken an oath to support the Constitution. The lawsuit argues that Trump violated his oath through his connection to the Jan. 6, 2021, riot at the U.S. Capitol.
Legal challenges to Trump's eligibility as a 2024 presidential candidate have gained momentum in some states, with top election officials in Arizona, Minnesota, New Hampshire and others weighing those concerns as they prepare state ballots for next year's Republican presidential primaries.
Trump has dismissed efforts to remove his name from ballots, calling them "nonsense and "election interference."