"If people tell you it's not a gateway drug - it's baloney," Colorado state Representative Kathleen Conti, a Republican, told two dozen parents and local residents at a recent meeting in the Denver suburb of Greenwood Village.

Conti described her own son's descent from pot smoking to heroin use at the get-together at a spacious private house, where listeners sat on dining room chairs in neat rows and munched on snacks, and organizers laid out pamphlets with titles such as "Put Colorado's Kids First."

Grassroots meetings are the main tool for groups seeking to defeat pot legalization in Washington state, Oregon and Colorado. With polls showing public support for legal pot and backers of the measures donating millions of dollars to the cause, opponents are struggling to connect with voters.

No U.S. state has approved legal recreational use of marijuana, although 17 states have legalized pot use for medical reasons.

Measures on the November 6 ballot in Washington state, Colorado and Oregon would allow sales of pot to those 21 and over at special stores regulated and taxed by the state.

A Colorado group leading the anti-legalization campaign there has raised less than $200,000 this year, according to campaign records. That is the most of any organization in the three states seeking to defeat the pot measures.

Roger Sherman, the Colorado group's campaign director, says he does not expect to buy air time on major media, and will rely instead on small meetings to get out the group's message.

"Talking about the impact to kids is critical and crucial to our success," Sherman said. "Suburban women are one of our strongest core constituencies for our campaign."

The groups are making their case about the risk to youths even though the ballot measures would bar pot sales to teenagers. There is conflicting data on whether making pot legal for adults would lead to increased use by minors.

The anti-legalization groups got a boost this week when the Colorado Education Association - the teachers' union in the state - came out against the measure, arguing it would hurt students.

Also last month, a major study showed chronic pot use among teens led to an average eight point decline in IQ.

The study by Duke University and King's College London, based on a decades-long survey of over 1,000 New Zealanders, added to a body of research showing harmful effects of pot on the developing brain.

"The bottom line is, if you care about young people succeeding in education and later in life in your state, then you don't want to legalize marijuana," said Kevin Sabet, a former adviser to Gil Kerlikowske, the Obama administration's drug policy director.

Sabet, who is working with anti-legalization groups in the three states, said the risk to young people has emerged as a focal point of their campaign.

The federal government considers marijuana a dangerous, illegal narcotic.

PROHIBITION?

Mason Tvert, co-director of the Campaign to Regulate Marijuana Like Alcohol, which is the main group behind the legalization measure in Colorado, said voters see prohibition as the "worst possible policy" for protecting teenagers.

"Because it's putting marijuana in an underground market, where it's entirely uncontrolled," he said.

His group points to a survey by the federal Centers for Disease Control and Prevention that showed while pot use went up nationwide, the percentage of teen users in Colorado fell to 22 percent in 2011 from 25 percent in 2009.

That coincided with the creation of a state agency in 2010 to oversee medical marijuana use, which Tvert's group says shows that regulating pot - not prohibiting it - may reduce teen use.

But opponents said there is no information to back that up.

A survey of U.S. teens released this year by the National Center on Addiction and Substance Abuse at Columbia University found 27 percent of respondents said it was easiest to obtain cigarettes, 24 percent said beer and 19 percent named marijuana.

"The claim that legalization and tight regulation will mean that youth will use less is bogus, because we don't have the experiment," said Rosalie Pacula, co-director of the RAND Drug Policy Research Center. She added that state-level legalization itself would be "the experiment."

"We don't know definitively that legalization will increase youth use. I can tell you from the research I've done, it supports such a conclusion," she said.

Marijuana is the most commonly used illicit drug in the United States. Teens are increasingly turning to it, and last year pot overtook cigarettes in popularity with 6.6 percent of 12th-graders using it daily, according to the University of Michigan Monitoring the Future Survey.

In comparison to the limited funds raised by pot opponents, the coalition of groups campaigning to legalize marijuana in Colorado have raised over $1 million this year, according to campaign finance reports. In Washington state, records show the pro-legalization group has taken in about $2 million since the measure qualified for the ballot in January.

In Oregon, the pot legalization group has only raised about $13,000, but the latest state records do not show any funds raised by opponents.

An Angus Reid Public Opinion Poll released in June found 52 percent of Americans support legalizing pot. The survey of 1,017 adults had a margin of error of plus or minus 3.1 percent.

(Editing by Greg McCune and Vicki Allen)

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