An amendment that would make it legal in Colorado for individuals to possess and for businesses to sell marijuana for recreational use has passed.

The Denver Post made the call at approximately 9:15 p.m.

Amendment 64 led with 52.7 percent voting yes and 47.3 percent voting no, with 1,507,746 votes or more than 50 percent of active voters counted, according to the Colorado Secretary of State’s office. The office said 25 counties had reported.

The early results prompted cheers throughout Casselman’s, a downtown Denver bar where hundreds of supporters were gathering.

Mason Tvert, co-director of Yes on 64, said the crowd was cautiously optimistic.

“We’re not going to jump to any conclusions just yet,” he said. “But we believe Colorado voters have decided to take a more sensible approach to how we deal with marijuana in the state.”

If the amendment passes, Colorado could become the first state to legalize recreational use of the drug, possibly clearing the way for creation of a marijuana industry. There are similar measures before voters in Washington and Oregon.

The ballot measure to amend the state constitution has fostered a national discussion about marijuana policy. Supporters hope approval would place pressure on the federal government to end marijuana prohibition everywhere. But critics say it could make Colorado a destination for drug tourists and prompt a federal crackdown.

“It’s unprecedented,” said Jonathan Caulkins, a Carnegie Mellon University professor whose research focuses on marijuana legalization. Implications would be “huge and impossible to pinpoint,” he said, and would put Colorado to the left of the Netherlands when it comes to marijuana policy.

The amendment would allow those 21 and older to purchase up to one ounce of the drug at specially regulated retail stores. Possession would be legal, but not public use. Adults could grow up to six marijuana plants in their homes.

Proponents have said the measure would not only divert profits from dangerous drug cartels to legitimate businesses, it would also generate tens of millions of dollars in tax revenues for state and local governments.

Opponents say the revenue boost is uncertain and that approval would make Colorado a tourist destination for illegal drug dealers. They also say Amendment 64 would lead to greater use of the drug by children, and to more drugged driving.

Campaigns fighting for and against the measure have raised nearly $4 million this election season, and the latest Denver Post poll showed it had support of 50 percent of voters, while 44 percent opposed it.

“We’ve been working over the past several years to build support among Colorado voters for ending the failed marijuana prohibition,” said Tvert.

The amendment leaves questions about its enforcement and long-term ramifications.

State criminal penalties for possessing the drug would disappear as soon as the election is certified, which could take up to two months.

The first recreational stores would be slated to open in January 2014 and would be separate from already existing medical marijuana dispensaries. Local governments could ban marijuana sales, and employers could still bar employees from using the drug.

The amendment doesn’t spell out the details of how the commercial marijuana industry will be regulated. It leaves that up to the state Department of Revenue, which would oversee the specialty shops. Proponents envision something similar to the state’s system governing medical-marijuana businesses, which involves security requirements, the monitoring of plants as they are grown and shipped and auditors who perform site checks.

There’s also the question of how the federal government will respond. A state law legalizing marijuana is a direct challenge to federal drug law, which regulates it as an illegal substance. Caulkins said federal agents could arrest individual users, though that would be a departure from their usual focus on large-scale dealers.

“It’s really anyone’s guess how the federal government will handle this,” he said.

The Colorado U.S. attorney’s office has said little about what it might do. A spokesman said the office would not change its approach to enforcement, which for medical marijuana has included ordering dispensaries near schools to shut down and raids on retailers who allegedly sell to the black-market.

In the year before the first stores open, the federal government may choose to challenge the measure in court by arguing that it is pre-empted by federal laws against pot.

“The implementation issues are huge,” said Roger Sherman, director of the No on 64 Campaign. He noted that because the measure is a constitutional amendment, it would have to go back to voters for repairs if there are problems. The legislature would be unable to intervene. “This is just wrong for Colorado. It’s not something Colorado needs to be known for.”

Seventeen states had efforts to put a marijuana legalization measure on the ballot in 2012 and just three succeeded. Voters in Washington and Oregon vote on similar proposals on Tuesday.

Sadie Gurman: 303-954-1661, sgurman@denverpost.com or twitter.com/sgurman