In the most serious challenge yet to Colorado's cannabis legalization policy, the states of Nebraska and Oklahoma took the unusual move of filing a case against it directly with the US Supreme Court. The two states argue that "the State of Colorado has created a dangerous gap in the federal drug control system... Marijuana flows from this gap into neighboring states, undermining Plaintiff States' own marijuana bans, draining their treasuries, and placing stress on their criminal justice systems." Lawyers for the Cornhusker State and the Sooner State assert that local authorities have registered a big uptick in cannabis entering their towns since Colorado legalized with voter-approved Amendment 64 in 2012. The suit also claims Colorado's legalization policy violates the Supremacy Clause of the US Constitution.