The Obama administration has largely turned a blind eye to the 17 states that currently flout federal drug law by allowing people with certain medical conditions to use pot, something banned under federal law.
"In a situation like this, where our law is at loggerheads with federal law, my primary job is to listen first," Colorado Gov. John Hickenlooper said Wednesday.
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.
Meanwhile, marijuana activists are waiting for a Washington, D.C., federal appeals court to decide whether marijuana should be reclassified from its current status as a dangerous drug with no accepted medical use.
Last year, the Drug Enforcement Administration rejected a petition by medical marijuana advocates to change the classification, which kept marijuana in the same category as drugs such as heroin. Reclassification of marijuana could open the door for more research into its medical use, so marijuana activists consider reclassification a major step.
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.
In Washington state, marijuana possession of an ounce or less would become legal on Dec. 6 if the measure is not blocked, though setting up a state-run sales operation would take a year.
Colorado's Republican attorney general, John Suthers, is a vocal critic of marijuana legalization. But he has vowed to defend Colorado's constitutional amendment in a legal battle with the federal government.
Marijuana business owners gathered in Denver for a two-day conference cheered the votes — but kept a close eye on their smartphones for word of a federal response.
Denver attorney Robert Corry told the crowd at the National Marijuana Business Conference that President Barack Obama sent a signal about marijuana when he made a record number of campaign visits to the battleground state this year and never talked about the marijuana vote.
"Silence equals consent. The time for the federal government to talk about marijuana legalization is over. The election has passed," Corry said to applause from the crowd. "Marijuana got 50,000 more votes than Obama on Tuesday. Don't think they didn't notice that."
But Corry quickly added that the federal marijuana approach will ultimately decide the question.
"Let's face it: the federal government is a fearsome force. They have the guns, they have the jails, they have the power," he said.