Nebraska and Oklahoma challenged neighbouring Colorado's recreational marijuana laws in the US Supreme Court on Thursday amid complaints its pot was seeping across their borders, and Colorado vowed to defend its laws.
Nebraska Attorney General Jon Bruning said he joined Oklahoma in filing the action against Colorado, where voters chose to legalize recreational marijuana in a landmark 2012 vote even as the drug remains federally outlawed.
"Federal law undisputedly prohibits the production and sale of marijuana," Bruning said in a statement, adding that drugs threaten the health and safety of children, and calling narcotics trafficking a national, interstate problem.
The lawsuit accuses Colorado of creating "a dangerous gap" in the federal drug control system.
"Marijuana flows from this gap into neighbouring states, undermining plaintiffs states' own marijuana bans, draining their treasuries, and placing stress on their criminal justice systems," the lawsuit said.
Colorado Attorney General John Suthers responded that because neighbouring states had expressed concern about Colorado-grown pot crossing their borders, he was "not entirely surprised" by the legal challenge.
"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," Suthers said in a statement. "We believe this suit is without merit and we will vigorously defend against it in the US Supreme Court."
The move comes amid a growing momentum for marijuana legalization after voters in Alaska and Oregon opted last month to allow the adult recreational use of marijuana, joining Colorado and Washington state.
Washington's Attorney General Bob Ferguson said he was disappointed by Nebraska and Oklahoma's move, and he vowed to "vigorously oppose any effort by other states to interfere with the will of Washington voters."
While marijuana is classified as an illegal narcotic under federal law, President Barack Obama's administration has given individual states leeway to frame their own rules.
Mike Elliot of the Colorado-based Marijuana Industry Group said Coloradans made their preference for licensed, regulated pot businesses clear at the ballot box.
"If Nebraska and Oklahoma succeed, they will put the violent criminal organisations back in charge," Elliot said.
Kevin Sabet, co-founder of anti-legalization group Smart Approaches to Marijuana, backed the legal action, saying while states should be able to decide appropriate sentencing and criminal sanctions, uniform federal drug laws are vital.
"Colorado's decisions regarding marijuana are not without consequences to neighbouring states, and indeed all Americans," Sabet said.