Colorado officials and marijuana advocates on Wednesday looked toward an imminent confrontation with the federal government one day after voters in the state endorsed a measure to legalize marijuana for recreational use.

Gov. John Hickenlooper said he is trying to speak soon with U.S. Attorney General Eric Holder to learn how the Justice Department will respond to the legalization measure’s passage.

Colorado Attorney General John Suthers said, despite his opposition to legalization, he would work with the state legislature to implement the new law — which he doubted the federal government would allow to stand. Proponents of Amendment 64, the measure voters approved with nearly 55 percent support on Tuesday, said they were optimistic the federal government would “respect the will of Colorado voters.”

And the Colorado U.S. attorney — the top federal prosecutor in the state — remained largely mum on how the conflict would play out. In a statement, local U.S. attorney’s office spokesman Jeff Dorschner reiterated that the Justice Department’s intent to enforce the federal law that makes all marijuana possession or sales a crime “remains unchanged.”

“My sense is that it is unlikely the federal government is going to allow states one by one to unilaterally decriminalize marijuana,” Hickenlooper said, adding, though, “You can’t argue with the will of the voters.”

What lies ahead for Colorado — after it and Washington on Tuesday became the first states in the nation to buck federal law by legalizing marijuana for any purpose — is largely unknown territory. No state since the beginning of marijuana prohibition has rolled back restrictions on cannabis to the extent Colorado now has.

Amendment 64 allows people 21 and older to possess up to an ounce of marijuana and grow up to six plants in their homes. The law won’t take effect, though, until Hickenlooper issues a proclamation certifying the vote. That will likely take at least a month. By law, it doesn’t have to happen until Jan. 5.

Until then, all non-medical marijuana possession and cultivation in Colorado remain a crime. In other words, at least for a few more weeks, the time to puff has not yet come to pass.

Amendment 64 also creates a system in which marijuana could be sold at specially regulated retail stores — which would be separate from medical-marijuana dispensaries. The details of that system must still be worked out by the state legislature and the Department of Revenue. The first recreational marijuana stores would likely not open until 2014, said Brian Vicente, one of Amendment 64’s proponents.

Before that happens, the federal government might choose to intervene by, among other options, filing a lawsuit arguing that the law’s retail sales section violates the U.S. Constitution because it frustrates federal drug laws.

“Coloradans should not expect to see successful legal challenges to the ability of the federal government to enforce its marijuana laws in Colorado,” Suthers wrote in a statement. “Accordingly, I call upon the United States Department of Justice to make known its intentions regarding prosecution of activities sanctioned by Amendment 64.”

The measure’s supporters said they are optimistic the federal government will allow the law to stand, and they heralded their victory Tuesday as the first step in a nationwide push to end marijuana prohibition.

“Things are moving,” Mason Tvert, one of the campaign’s chief proponents, said. “They’re moving quickly. We think the writing is on the wall.”

Amendment 64 — which polls showed hovering at around 50 percent support heading into Election Day — won with what even supporters said was a surprising amount of cushion. With roughly 1.28 million votes in its favor, it drew more support than President Barack Obama did in winning Colorado.

It passed in more counties than it lost — 33 to 31. It won in seven counties that voted for Republican Mitt Romney and lost in only one — Conejos — that voted for Obama.

“We are at the tipping point on marijuana policy,” Vicente said. “This is an area where our voters and our citizens are really leading.”

Drug-abuse prevention professionals, though, said Wednesday that Colorado is going down a dangerous path. They predicted marijuana legalization would increase pot use, especially among young people, and lead to higher rates of drugged driving and substance abuse.

“We need to let people know it is not OK for youths to use marijuana,” said Christian Thurstone, a substance-abuse treatment doctor at Denver Health medical center. “We need them to realize it’s not OK for young people to drive under the influence of marijuana.”

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold