Former Vice President Joe Biden is the only leading Democratic presidential candidate to oppose the federal legalization of marijuana.

Speaking at a town hall in Las Vegas Saturday, Nov. 16, Biden said there’s not enough evidence to prove “whether or not it is a gateway drug.” Biden said states should be able to “make a judgment to legalize marijuana,” like Michigan voters did for medical cannabis in 2008 and recreational use in 2018.

“Nationally I’m not prepared to push for the legalization,” Biden said. “Medical marijuana, yes. The legalization of marijuana for recreational use ... I need more data to make that judgment. But, no one should go to jail for it, period.”

WATCH: @JoeBiden on marijuana legalization - "There is not nearly been enough evidence...as to whether or not it is a gateway drug," he said last night, "It’s a debate." Biden said states should decide on legalization & that he supports medical marijuana. @CBSNews pic.twitter.com/s4CE32phLS — Bo Erickson CBS (@BoKnowsNews) November 17, 2019

Public opinion has experienced a decades-long shift on the legalization of marijuana. Pew Research polling shows a majority of Americans believe marijuana should be legal, including 54% of Baby Boomers, a demographic Biden has strong appeal with.

Michigan became the 10th state, first in the Midwest, to legalize cannabis for recreational use through a ballot initiative supported by 56% of voters. Recreational marijuana will be available for purchase in Michigan months before voters cast a ballot in the March 2020 Democratic primary.

Biden is the leading Democratic candidate in Michigan, according to recent polling of likely primary voters. However, some polls indicate stronger support for U.S. Sen. Elizabeth Warren, D-Mass., who said she would make marijuana legal if elected.

Biden said “legalization is a mistake” in 2010 and was asked whether his position changed at the Las Vegas town hall.

Biden said he would change marijuana from a Schedule 1 drug, defined as drugs with no accepted medical use and a high potential for abuse, to a Schedule 3 drug, defined as having a moderate to low potential for addiction.

Lowering the classification of marijuana would make it easier to research. Scientists need federal approval to conduct research with Schedule 1 drugs.

“I want to make sure we know a lot more about the science behind it,” Biden said. “It is not irrational to do more scientific investigation to determine, which we have not done significantly enough, whether or not there are anythings to relate to whether it’s a gateway drug."

Further research is needed to explore this question, according to the National Institute on Drug Abuse, a federal research organization.

Several Democratic candidates have said medical marijuana use could provide benefits compared to opioids, particularly because it is less addictive.

There is evidence to suggest early marijuana use could make people vulnerable to addiction to other drugs. However, the NIDA determined, the majority of people who use marijuana don’t go on to use other substances.

Biden does call for decriminalizing marijuana and automatic expungement for marijuana convictions as part of his criminal justice reform agenda. However, other Democrats said Biden’s plan doesn’t go far enough.

U.S. Sen. Cory Booker, D-N.J., said any meaningful criminal justice reform includes legalizing marijuana. Booker introduced a bill in the Senate to end the federal prohibition on marijuana earlier this year.

Booker’s bill was co-sponsored by other presidential hopefuls, including Warren and Sens. Bernie Sanders, I-Vt.; Kamala Harris, D-Calif.; and Michael Bennet, D-Colo.

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