DENVER - Colorado voters legalized recreational pot use on Tuesday, setting up a clash with federal drug policy. A similar measure was also approved in Washington state.
Another initiative in Oregon appeared headed for defeat.
When state and federal laws conflict, federal law takes precedence. Federal authorities could sue in an attempt to block the measures in Colorado and Washington from taking effect.
It remained unclear how the federal government would respond.
Colorado's direct challenge to federal drug law comes 12 years after the state legalized marijuana for people with certain medical conditions.
The state's latest pot measure stated that adults over 21 could possess up to 1 ounce of marijuana, or six marijuana plants, for personal use.
The amendment also allows commercial sales of the drug starting next year, though cities would be free to prohibit commercial pot businesses. It specifically prohibits using the drug "openly or publicly."
Legalization backers argued that marijuana is no more dangerous than alcohol and should be regulated and taxed. Opponents said legalizing pot would increase its use and make it easier for youths to obtain it.
Colorado exit polls showed the measure was solidly supported by thirty-somethings but opposed by about two-thirds of those older than 65.
Shelby Jensen, 22, of Lafayette, was among those supporting the proposal.
"There's a lot bigger issues to focus on. They're spending too much time and money prosecuting people caught with marijuana," said Jensen, a barista who is registered as unaffiliated.
The amendment sets up an elaborate regulatory scheme for how the drug could be used and sold. The amendment directed state lawmakers to regulate the drug through the Department of Revenue, which already oversaw alcohol sales and medical marijuana dispensaries.
The measure also directed state lawmakers to place an excise tax of up to 15 percent on marijuana sales, with the first $40 million each year devoted to school construction.
After approval by the Legislature, the pot tax would face final approval by voters.
The amendment also allows the production of industrial hemp, a far less potent cousin of marijuana that can be used for fiber and fuel, among other things.
Nine states - Hawaii, Kentucky, Maine, Maryland, Montana, North Dakota, Oregon, Vermont and West Virginia - have already passed laws allowing hemp cultivation or research.
But federal drug law prohibits hemp production, so industrial hemp used in products from granolas to soaps is currently imported.
Colorado's amendment did not affect existing medical marijuana law.
Colorado has 536 dispensaries licensed statewide. The state had several dozen more before a federal crackdown began earlier this year, targeting dispensaries near schools.
Colorado voters rejected marijuana legalization in 2006, 59 percent to 41 percent.