Voters in Colorado and Washington on Tuesday approved measures allowing adults to use marijuana for any purpose, NBC News projected, marking an historic turning point in the slow-growing acceptance of marijuana usage.

In Massachusetts, voters also approved an initiative allowing people to use marijuana for medicinal purposes, NBC News projected. In Arkansas, a similar initiative failed, according to NBC News projections.

In all, voters in six states were being asked to decide on a wide array of laws around legalizing marijuana for medicinal or recreational purposes.

In three of those states – Colorado, Oregon and Washington – voters were deciding whether to allow people over 21 to use marijuana for any purpose. In Oregon, NBC News projected that the initiative to legalize marijuana had failed.

In Montana, NBC News projected that voters had approved a plan to to revamp an existing medicinal marijuana law to make it more restrictive.

The laws legalizing marijuana for recreational or other purposes could face federal challenges, because marijuana possession is still a federal crime. But so far, the Justice Department has declined to discuss how it might react if the laws pass. Late Tuesday, a spokesman said in an e-mail that they were reviewing the Colorado initiative and had no immediate comment.

Proponents say it’s about time pot was made legal and that it would create new avenues of tax revenue. But opponents say legalization would lead to more drug abuse and concerns about things like driving while impaired.

Opponent Kevin Sabet, a former senior advisor to the Obama administration and an assistant professor at the University of Florida’s college of medicine, said he was expecting legal challenges at the state and federal level.

“This is just the beginning of the legalization conversation, so my advice to people who want to toke up legally or think that they can buy marijuana at a store tomorrow is that we’re a very long way from (that),” Sabet said.

Proponents of the legislation also said they expected some legal wrangling.

“It sets up a clear and obvious challenge with the federal government,” said Allen St. Pierre, executive director of NORML, which has fought for years to legalize cannabis.

But proponents also were celebrating what they saw as a turning point in a long-running battle to make marijuana more available to the general public.

“We are reaching a real tipping point with cannabis law reform,” said Steve DeAngelo, a longtime advocate for legalizing marijuana and the director of the nation’s largest medical cannbabis dispensary, Harborside Health Center in Oakland, Calif.

Colorado Gov. John Hickenlooper acknowledged legal challenges but said the state would work to resolve the conflict between federal and state laws.

"It's probably going to pass, but it's still illegal on a federal basis. If we can't make it legal here because of federal laws, we certainly want to decriminialize it,” he told NBC’s Brian Williams.

Seventeen states and the District of Columbia already have laws allowing for the medical use of marijuana, according to the National Council of Legislatures.

The initiatives in Washington, Oregon and Colorado would take things one step further, explicitly allowing people to smoke pot for more than just medicinal purposes.

The idea of legalizing marijuana has gained acceptance in recent years. A Gallup poll released in October of 2011 found that 50 percent of Americans now favor legalizing pot. A decade ago, only around 34 percent were in favor. Liberals and adults under 29 are the most likely to approve of legalizing use of the drug.

Here’s a look at the states considering marijuana laws Tuesday.

Arkansas: Voters in Arkansas will consider whether to legalize marijuana for medical purposes.

Colorado: Voters in Colorado are being asked to approve a bill that would allow people 21 and over to possess and use a small amount of marijuana for recreational purposes. A similar measure was defeated in 2006.

Massachusetts: Voters in Massachusetts are being asked to vote on whether it’s OK to legalize marijuana for medical purposes.

Montana: In 2004, voters in Montana approved a law allowing marijuana for medical purposes. Then, in 2011, the legislature approved replacing it with a new, more restrictive one. Voters on Tuesday will decide whether to allow those restrictions to be upheld.

Oregon: Voters in Oregon are being asked to decide whether to legalize marijuana use for people who are 21 years or older, and to tax and regulate it in the same way as alcohol.

Washington: The Washington bill would allow people over age 21 to possess a small amount of pot for personal use.

More election coverage from NBCNews.com:

• Obama wins re-election; Ohio, Iowa, Wisconsin prove pivotal
• Democrats gain in Senate with wins in four states
• Rape remarks sink two Republican Senate hopefuls
• In costliest-ever Senate race, Warren beats Brown for Mass. seat
• Republicans to maintain control of House, NBC News projects
• Colorado, Washington approve recreational marijuana use
• In 11 governor races, it's about jobs and taxes
• Majority of voters see American on wrong track

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