The governor of Colorado said he planned to talk by phone with U.S. Attorney General Eric Holder about the measures that contradict federal law banning the use of pot.
Seventeen states and Washington, D.C., already allow marijuana use by people with certain medical conditions. Still, federal drug law outlaws use of the drug in all circumstances.
Voters in Colorado and Washington pushed the limits even further when they approved ballot measures Tuesday allowing adults over 21 to possess small amounts of marijuana under state regulation and taxation.
Colorado Gov. John Hickenlooper has said Colorado will respect the will of voters but added that he was awaiting word from the U.S. Department of Justice on how to proceed.
"In a situation like this, where our law is at loggerheads with federal law, my primary job is to listen first," the governor said.
Hickenlooper opposed the ballot measure and has downplayed the likelihood of a commercial marijuana market materializing in Colorado.
"Based on federal law, if it’s still illegal under federal law, I can’t imagine that 7-Eleven is ever going to sell it," he said.
Marijuana advocates hope the federal government maintains its current posture of mostly ignoring states that flout federal law by allowing medical use under certain circumstances.
The U.S. government has cracked down during the past two years on more than 500 marijuana dispensaries in several states, but no one has faced federal prosecution for personal use.
"It would certainly be a travesty if the Obama administration used its power to impose marijuana prohibition upon a state whose people have declared, through the democratic process, that they want it to end," said Brian Vicente, co-author of Colorado’s marijuana measure.
Earlier this week, Justice Department spokeswoman Nanda Chitre said enforcement of the federal Controlled Substances Act remained unchanged.
"In enacting the Controlled Substances Act, Congress determined that marijuana is a Schedule I controlled substance," Chitre said. "We are reviewing the ballot initiative and have no additional comment at this time."
Eric Brown, a spokesman for Hickenlooper, would not say whether the governor planned to disclose the details of his call with Holder.
If Colorado’s marijuana ballot measure is not blocked, it would take effect by Jan. 5, the deadline for the governor to add the amendment to the state constitution. The measure allows adults to possess up to an ounce of marijuana, and six marijuana plants, though public use of the drug and driving while intoxicated are prohibited.
Colorado’s measure also directs lawmakers to write regulations on how pot can be sold, with commercial sales possible by 2014.