Colorado Gov. John Hickenlooper said Wednesday he is trying to speak with U.S. Attorney General Eric Holder about how the federal Justice Department will respond to Colorado’s legalizing of marijuana.

Hickenlooper said he hopes to set up a phone call with Holder as early as Thursday to hear the federal government’s position. He said he doubted the Justice Department would respond passively to the passage of Amendment 64, which legalizes limited possession and commercial sales of marijuana.

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

Hickenlooper said understanding the federal response will be vital as state lawmakers and officials look at how to move forward with new regulations for marijuana stores. Those regulations need to be in place by January 2014, meaning lawmakers will have only one legislative session — which starts in about two months — to work them out.

“If the attorney general of the United States is willing to allow that to happen, and I would certainly point out the reasons why that makes sense, certainly then we have a longer discussion,” Hickenlooper said. “Then we get back to looking at what are the details, how would we tax this, how would we create a regulatory environment that was robust and really held people accountable?”

Colorado voters on Tuesday approved Amendment 64 with nearly 55 percent support, joining Washington as the first states in the nation to legalize marijuana use and retail sales. In what had been projected to be a close race, the measure ultimately received more votes than President Barack Obama.

Hickenlooper’s statements came as proponents and opponents of the measure held dueling news conferences to address the fall-out from Tuesday’s historic vote. The measure’s supporters heralded their victory 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.”

Meanwhile, opponents of marijuana said Colorado voters will come to regret the consequences of passing the amendment. They predicted marijuana legalization would increase pot use, especially among young people, and lead to higher rates of drugged driving and substance abuse.

“This is contrary to the interest of public health,” said Bob DuPont, founding director of the National Institute of Drug Abuse.

Amendment 64 allows people 21 and older to possess up to an ounce of marijuana and grow six plants in their homes. It also would allow marijuana to be sold at specially regulated stores, which local governments could ban from their communities.

The law will not take effect until Hickenlooper signs off on the vote, a process that could take a month or more. Until then, it remains illegal to possess any amount of marijuana in Colorado absent medical-marijuana approval. Recreational marijuana stores would not start popping up until 2014, after regulations for them are crafted.

The federal government may go to court to block the new law from being implemented, but officials have not yet said precisely how they would respond.

“The Department of Justice’s enforcement of the Controlled Substances Act remains unchanged,” local U.S. attorney’s office spokesman Jeff Dorschner wrote in a statement. “In enacting the Controlled Substances Act, Congress determined that marijuana is a Schedule I controlled substance. We are reviewing the ballot initiative and have no additional comment at this time.”

Amendment 64 proponent Brian Vicente said he hopes there will not be a crackdown.

“We are cautiously optimistic the federal government will respect the will of Colorado voters,” Vicente said.

But Hickenlooper isn’t so sure.

“Some people think there’s going to be big shops, and there’s going to be large (growers),” Hickenlooper said. “It’s hard for me to imagine that happening and having big pot shops if the federal government still views it as illegal.”

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