Gary Cameron / Reuters Sen. Chuck Grassley (R-Iowa) is one of the co-chairs of the drug caucus, which hosted an anti-marijuana hearing on Tuesday.

A Senate hearing on state marijuana laws Tuesday was biased and dishonest, focusing mostly on debunked claims that marijuana use leads to substances like heroin and cocaine, drug policy groups said.

"These hearings are a one-sided sham with the deck stacked with witnesses who have a track record of vehemently opposing marijuana legalization," said Michael Collins, Drug Policy Alliance deputy director. "An honest evaluation of marijuana legalization would include the undeniable benefits of legalization, like the massive drop in marijuana arrests, the billions in taxes, and the transition from an underground market to a regulated one."

Sens. Dianne Feinstein (D-Calif.) and Chuck Grassley (R-Iowa), co-chairs of the Caucus on International Narcotics Control, called the hearing to evaluate a Department of Justice 2013 decision to not enforce federal drug laws in states that have legalized recreational marijuana use. Feinstein and Grassley, two of the Senate's most strident anti-drug crusaders, cited a recent Government Accountability Office report that criticized DOJ for inadequately monitoring the effects of state marijuana policies.

The four witnesses who testified at the hearing showed little diversity in their viewpoints: GAO's director for Homeland Security and Justice Jennifer Grover, who coauthored a report criticizing DOJ; California prosecutor Benjamin B. Wagner, who led raids on pot farms in California; Nebraska Attorney General Doug Peterson, who sued Colorado over its marijuana law; and Denver-based pediatrician Kathryn Wells, who sits on the board of Smart Approaches to Marijuana, a group opposed to legalization.

No one at the hearing spoke in favor of reforming marijuana laws. Pro-legalization groups chided Grassley for not holding hearings on Senate legislation that would legalize marijuana for medical use and re-schedule the substance as a less dangerous drug.

"Grassley may be stuck in the past, but the rest of us are not. It's time for him to do his job and hold real hearings on real legislation that could make a real difference for patients and providers across the country," said Aaron Smith of the National Cannabis Industry Association.

Sen. Jeff Sessions (R-Ala.) lamented Colorado's legalization of recreational pot, arguing the state had led the march toward national acceptance of pro-marijuana policies. He praised past prevention efforts, such as Nancy Reagan's "Just Say No" campaign, arguing the federal government once sent a message "that good people don't smoke marijuana."

"We need grown-ups in charge in Washington to say marijuana is not the kind of thing that ought to be legalized ... that it's in fact a very real danger," Sessions said.

Marijuana policy groups condemned Sessions' comments as old-fashioned.

"The senator’s views on marijuana are outdated and clearly out of touch with those of most Americans," said Robert J. Capecchi, Marijuana Policy Project director of federal policies. "Marijuana is less harmful than alcohol, and Sen. Sessions doesn’t seem to have any problem with adults who have a drink from time to time. His comments amount to nothing more than the last gasps of reefer madness."

"There are millions of Americans who have used marijuana, including the last three presidents, gold medal-winning Olympic athletes, renowned scholars and celebrated musicians," Marijuana Majority chairman Tom Angell said. "Maybe Sen. Sessions just needs to get out a little and meet more Americans so he can realize that some good people -- in fact many good people -- also happen to be marijuana consumers."