Leaders of the anti-marijuana legalization movement gathered at The Heritage Foundation on Capitol Hill Tuesday to rally in opposition to the nation's slide toward relaxing cannabis laws.

Marijuana possession remains a federal crime, but most national polls show majority support for legalization and four states already have abolished penalties for adults 21 and older. Many others allow the drug for medical use or have lowered penalties.

Two House Republicans leading the charge to combat more permissive marijuana policies spoke at the event, attracting loud hecklers.

“I don’t want to listen to these lies,” a bearded heckler announced, walking out as Rep. Andy Harris of Maryland took the podium at the conservative think tank.

Harris, author of a budget rider that would have blocked a decriminalization law in the nation’s capital, said marijuana use can cause a drop in IQ and, he said, has no proven medical value.

“The people voted,” a second protester shouted.

An undeterred Harris continued: “We can’t have a discussion about legalization for adults” without considering the possible “spillover effect” on teenagers.

“Relaxing laws clearly leads to more teenage drug use,” he said, because “the message that it’s dangerous will be blunted.”

Thus far, that hypothesis hasn’t been proved in the first states to legalize marijuana use for adults. In Colorado, the state’s health department found high school marijuana use did not increase from 2011 – when pot was not legal – to 2013, the first full year of legalization in the state but before retail shops opened.

“This is about justice,” a third protester shouted on her way out the door.

As Harris shared that a pharmaceutical company leader expressed concern to him about Maryland’s decriminalization of marijuana, security guards worked to force out a man suspected of planning to interrupt.

“Don’t tread on D.C., respect the will of D.C. voters,” the man said after an argument with guards.

Harris laughed when asked by U.S. News about the hecklers, and declined to say if he found their arguments persuasive.

Harris and the event’s second speaker, Rep. John Fleming of Louisiana, have forcefully advocated using Congress’ oversight power to hamstring the District of Columbia’s local marijuana regulations, which include a legalization ballot measure that overwhelmingly passed in November.

Harris’ effort to block decriminalization in the nation's capital failed – the law took effect in July – after he conceded the district could respond by ending enforcement of all laws against marijuana.

Fleming tells U.S. News he’s unsure if the forthcoming “cromnibus” spending deal being negotiated behind closed doors by congressional leaders will include language blocking legalization in the district. But he says he hopes it does.

The National Journal reported Tuesday afternoon the spending deal will allow the district initiative to take effect, but that it also will ban the city from crafting regulations to allow for recreational marijuana stores.

Fleming told the anti-marijuana gathering the drug is a “highly addictive substance” and said it’s a gateway to more serious drug use. “Humans do not just decide they’re going to go out and use meth,” he said, adding that "more likely than not," meth users first smoked pot.

Legalization supporters generally argue that street dealers of marijuana can introduce clients to other drugs, which would not happen with aboveboard pot sales.

Fleming, a medical doctor like Harris, said he considers many claims about the medicinal benefits of marijuana to be “complete rubbish.”

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“My libertarian friends suggest to us the old motorcycle helmet argument,” he said, referencing the argument that the government has no business regulating the personal choices of individuals. Fleming countered with his opinion that “we as taxpayers have a vested interest in what society does.”

Fleming also warned that “if we continue to relax our laws on the acceptance of marijuana, we’re going to see greater addiction [and] greater government dependency.”

Event organizer Kevin Sabet, a former presidential drug adviser who co-founded the anti-legalization group Smart Approaches to Marijuana, encouraged a nuanced conversation about marijuana before introducing a panel including Barry McCaffrey, the Clinton administration’s drug czar, and Stuart Gitlow, a board member of Sabet’s group and a leader of the American Medical Association, which opposes legalization.

Gitlow said there is “no legitimate research at this time that the plant marijuana has medical value,” eliciting a heckler's response that research shows allowing marijuana for medical use may reduce painkiller overdose deaths. Gitlow said a “fairly small minority” of people suffer psychological issues after using marijuana, but that those in societies that do not promote drug use do not suffer addiction.

McCaffrey, meanwhile, said he’s upset that some legalization proponents have interpreted his blasé comments about adult marijuana use as an evolution of his beliefs.

“We’ve got a disaster facing us,” he said about marijuana legalization. “We’ve lost the message war on health … We’ve lost the political war … We’ve lost the media war.”

McCaffrey said “libertarians have a nice coherent argument" in favor of legalization, but that marijuana foes cannot allow either major party to embrace that position. He also scolded a CNN host who, he said, found marijuana a humorous topic.