DENVER — Two states — Oklahoma and Nebraska — filed a lawsuit Thursday against neighboring Colorado over its law legalizing recreational marijuana, contending that it violates the Supremacy Clause of the U.S. Constitution.

The legal challenge, an original action filed with the U.S. Supreme Court, is the first of its kind since Colorado voters passed Amendment 64 in 2012, which allows recreational pot use and sales for adults 21 and over.

Nebraska Attorney General Jon Bruning said one problem with Colorado’s law is that marijuana is now crossing state lines, causing headaches for states where pot remains illegal under both state and federal law.

“Colorado has created a system that legalizes, promotes and facilitates distribution of marijuana,” Mr. Bruning said in a statement. “The illegal products of this system are heavily trafficked into neighboring states, causing an unnecessary burden on the state of Nebraska. Colorado has undermined the United States Constitution, and I hope the U.S. Supreme Court will uphold our constitutional principles.”

Colorado Attorney General John Suthers announced that his office would defend Colorado’s law. Mr. Suthers is term-limited and will be succeeded in January by Cynthia Coffman, his deputy, who was elected in November. Both officials are Republicans.

Mr. Suthers argued that the responsibility to enforce federal laws such as marijuana prohibition outside Colorado is a federal responsibility not Colorado’s and his neighbors should take it up with Washington.

“Because neighboring states have expressed concern about Colorado-grown marijuana coming into their states, we are not entirely surprised by this action,” Mr. Suthers said. “However, it appears the plaintiffs’ primary grievance stems from non-enforcement of federal laws regarding marijuana, as opposed to choices made by the voters of Colorado. We believe this suit is without merit and we will vigorously defend against it in the U.S. Supreme Court.”

Oklahoma Attorney General Scott Pruitt said the flow of marijuana into his state has made it tougher to enforce anti-pot laws.

“As the state’s chief legal officer, the attorney general’s office is taking this step to protect the health and safety of Oklahomans,” Mr. Pruitt said in a statement.

Marijuana is banned under federal law as a controlled substance, but a Justice Department memo issued last year outlined enforcement priorities for states that legalize pot, including taking steps to keep the drug within the state borders.

Mr. Bruning said marijuana that hasn’t happened. In Sidney, Nebraska, just outside the Colorado border, police say half of all traffic stops result in marijuana arrests, according to KLKN-TV in Lincoln.

“This contraband has been heavily trafficked into our state,” Mr. Bruning said at a news conference in Lincoln. “While Colorado reaps millions from the production and sale of pot, Nebraska taxpayers have to bear the cost.

Colorado’s retail pot market is approaching its first anniversary in January. Colorado voters approved medical marijuana in 2000, but the lawsuit only challenges recreational pot.

Colorado Gov. John Hickenlooper told the Denver Post that he has discussed such concerns with officials in Nebraska and Oklahoma, adding, “I’m not sure filing a lawsuit is the most constructive way to find a solution to whatever issues there are.”

Washington state also legalized recreational pot in 2012, but that retail market didn’t open until June.

Voters in Alaska, Oregon and the District of Columbia passed measures in November that legalize the recreational use of marijuana, although Congress’s de facto veto over the District’s laws means the nation’s capital isn’t likely to see legalized pot any time soon.

⦁ This article was based in part on wire-service reports.

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