Former Vice President Joe Biden says he won’t legalize marijuana on the federal level because there’s not “enough evidence” on whether it’s a gateway drug, and a decision on legalization should be at the state level.

The 2020 candidate for the Democratic presidential nomination said during a town hall in Las Vegas he supports medical marijuana and doesn’t think marijuana possession should be a crime. During a speech in June, Biden said he supports expunging the records of those with possession convictions.

“The truth of the matter is, there’s not nearly been enough evidence that has been acquired as to whether or not it is a gateway drug,” Biden said Saturday. "It's a debate, and I want a lot more before I legalize it nationally. I want to make sure we know a lot more about the science behind it."

The comments came ahead of Wednesday's scheduled markup of a marijuana legalization bill Wednesday by the House Judiciary Committee.

Though Biden said more scientific investigation of marijuana is needed, a 20-year-old study by the Institute of Medicine found no evidence that marijuana use leads to use of harder drugs even though smoking pot generally preceded  more destructive behavior. The National Institute on Drug Abuse said research indicates marijuana users rarely go on to harder drugs but there is a correlation with alcohol abuse. The research, which was updated in September, also indicates THC – the psychoactive chemical in marijuana -- can prime the brain for substance abuse.

“States should be able to make a judgment to legalize marijuana,” he said.

Biden’s campaign has said he’s willing to reclassify marijuana as a Schedule II drug, It’s currently on the Schedule I list, which includes such substances as heroin and LSD.

Biden rivals Sens. Bernie Sanders of Vermont, Elizabeth Warren of Massachusetts and Cory Booker of New Jersey all have said they favor legalizing marijuana nationally. Also in favor of legalization are South Bend (Ind.) Mayor Pete Buttigieg, former Housing Secretary Julian Castro, Rep. Tulsi Gabbard of Hawaii, Sen. Kamala Harris of California, Sen. Amy Klobuchar of Minnesota and businessman Andrew Yang.

Thirty-three states and the District of Columbia have passed medical marijuana laws and 11 states and the District have legalized recreational use. A number of other states also have decriminalized possession of small amounts.