Three states will decide on Tuesday whether to take the unprecedented step of legalizing marijuana. NBC's Pete Williams reports.

Marijuana-legalization backers believe they’re well schooled on all things leafy – from cannabis to political tea leaves. With pro-pot measures leading in recent polls in Washington and Colorado, proponents don’t foresee federal agents interceding in those states if voters approve the initiatives.

Their rationale: Two years ago, when California voters considered a similar proposal to legalize the adult possession of an ounce or less of pot, U.S. Attorney General Eric Holder publicly vowed the feds would continue to prosecute anyone in that state caught possessing marijuana — even if the law passed. It failed.

This year, in contrast, federal anti-drug authorities have repeatedly declined to discuss decriminalization proposals in three states — including a measure in Oregon that would end the prohibition of marijuana there. (That initiative trailed in recent polls.) The response routinely delivered by U.S. Department of Justice spokeswoman Allison Price, including in an e-mail to NBC News: “We are not going to speculate on the outcome of the various ballot initiatives in each of the states.”

“That, to me, is significant because they didn’t just copy and paste what they did and said in 2010. We feel pretty good about that,” said Alison Holcomb, campaign director for Washington’s Initiative 502, which seeks to regulate and tax marijuana production and distribution in that state. According to a poll released Thursday, Initiative 502 had the support of 55 percent of Washington voters.

But Dr. Kevin A. Sabet, former senior drug policy advisor to the Obama Administration and director of the Drug Policy Institute at the University of Florida, predicts a far different law-enforcement reality on the ground in Washington — as well as in Colorado, where Amendment 64 would allow the state to regulate marijuana as it does alcohol.

“Once these states actually try to implement these laws, we will see an effort by the feds to shut it down,” Sabet said.

Sabet’s vision of post-election pot realities in Washington and Colorado — where Amendment 64 has majority support, according to a recent poll — seems to suggest a possible weed war between the feds and the states.

“We can only guess now what exactly that would look like,” Sabet said. “But the recent U.S. Attorney actions against medical marijuana portends an aggressive effort to stop state-sponsored growing and selling at the outset.” (That includes, he said, letters sent by federal prosecutors last January to medical marijuana dispensaries in Colorado operating within 1,000 feet of schools, ordering those businesses to halt sales.)

“The question voters should be asking themselves,” Sabet said, “before voting on these initiatives is this: Is your right to buy pot from a store down the street worth the risk of increased teenage drug abuse, increased enforcement action by the feds, and increased problems like 'stoned driving?’ "

Whether a legal showdown is ignited or not, some state-legalization proponents see their measures as possible footholds in a march toward national marijuana decriminalization.

“Exactly 80 year ago, Colorado voters approved a ballot measure to appeal alcohol prohibition, and that came prior to it being repealed by the federal government,” said Mason Tvert, co-director of the Yes on 64 campaign in Colorado, a state that already regulates the sale of medical marijuana. “And it was the individual states taking that type of action that ultimately resulted in the federal (Prohibition) repeal.

“The same kind of thing is underway with marijuana,” he added. “Whether there’s going to be a critical mass, who knows?”

In Washington, Holcomb echoed that uncertainty: "I'm not sure how that’s going to play out.”

“It may be there’s going to some generational evolution on this. Medical marijuana was introduced in the mid-90s and we were still talking to a lot of people that were coming out of the ‘Reefer Madness’ era, who had a lot of fear. And (medical marijuana) was a really powerful way to help them see that marijuana is not this terribly scary thing that they had been told,” Holcomb said.

Indeed, the most recent poll on Colorado’s Amendment 64 found that 73 percent of state state’s residents who are under age 30 want pot legalized. At the same time, more than half of seniors are against decriminalizing marijuana.

Anti-drug watchdog Sabet, meanwhile, sides with most current political leaders — “the overwhelming majority of Congress (and) both major presidential candidates” — as well as the American Medical Association standing against the decriminalizing of marijuana: “I don't envision national legalization as a realistic possibility in the near future.”

“The state-level efforts could soon prove to be a tipping point for more aggressive legalization initiatives,” Sabet said. “However, there is a growing consensus within the medical and treatment community — who deal with the problems of marijuana use and addiction everyday — to reject both extreme prohibition and lax legalization. I think we'll end up with a policy that is more centrist, for example, not punishing people by barring them from a job for a past marijuana arrest, but also not allowing marijuana to be marketed and sold like alcohol or cigarettes.”

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