Colorado joined two dozen other states on Wednesday, Aug. 26, in a lawsuit against the U.S. Postal Service to block a set of new rules for mail-in voting in the 2026 midterm elections. 

The lawsuit comes two days after the U.S. Supreme Court cleared a path for President Donald Trump to implement an executive order he signed on March 31 aimed at limiting mail-in voting. The order directs the U.S. Department of Homeland Security to compile a list of U.S. citizens based on federal databases and requires the Postal Service to send mail ballots only to voters on that list.

The order also places new requirements on how mail-in ballots can be sent to “enhance election integrity.” That includes mandating that states use unique identifiers on ballot envelopes, such as barcodes, “reducing the risk of fraud and protecting the integrity of Federal elections,” the order states.

Colorado Attorney General Phil Weiser, in a statement, called the Postal Service’s rule “the latest attempt by the Trump administration to exert federal control over elections, cause mischief ahead of the midterm elections, and seize states’ constitutional authority to administer elections.”

Weiser and several other states had already sued over the Postal Service’s rule when it was first proposed. That suit, filed in April, led to a federal judge on July 25 blocking the order from taking effect in those states.

The Trump administration later appealed to the Supreme Court, and on Monday, the court’s conservative majority lifted the lower court’s block on Trump’s order. The Supreme Court did not rule on the legality of the order itself, but on the legal process, saying that the states needed to wait until the order was finalized and ready for implementation, leaving the door open to further legal challenges. 

The two dozen states that sued on Wednesday argued the Postal Service’s rule, which is now final, violates the separation of powers in the U.S. Constitution, which gives states and Congress the authority to oversee elections and pass election laws, not the president. They also argued that the rule is inconsistent with federal voting rights law and violates the Privacy Act. 

“States determine the time, place and manner of elections – not the president,” said Weiser, a Democrat who is running for governor this fall. “And Congress has never given the (Postal Service) authority to regulate mail ballots. Most Coloradans vote using mail ballots, and I will continue to defend our state’s gold standard election system.”

Voting rights groups, along with local and state election officials, warn that the new rules will be cumbersome to implement so soon before the midterm elections and could disenfranchise voters. 

County clerks in Colorado can begin sending mail ballots to registered voters on Oct. 2, and have until Oct. 9 to send ballots.