William Cummings | USA TODAY

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Former Vice President Joe Biden said Saturday that more study is needed to determine whether marijuana is "a gateway drug," making him the only one of the leading Democratic presidential primary candidates to oppose legalization on the federal level.

Biden – who said in 2010, "I still believe it's a gateway drug," and "legalization is a mistake" – was asked at a town hall in Las Vegas if his position had changed.

"No, it hasn't changed," Biden said. He explained that although he supports allowing states to determine their own marijuana policies, "the truth of the matter is, there has not been nearly enough evidence acquired as to whether or not it's a gateway drug."

Though Biden opposes legalization on the federal level, he said anyone incarcerated for marijuana should be released and criminal records should be expunged of any marijuana charges. He supports removing marijuana from the list of Schedule 1 drugs, which says it has no medical use, to Schedule III, which would make it easier to research.

The gateway drug theory says marijuana use can lead to the use of harder drugs, such as heroin or cocaine, because it fuels an appetite to experiment with other substances and enters the user in a social circle where they are available. Critics of the theory say myriad factors contribute to addiction and no evidence has been found that pot's effects lead to hard drugs.

"It's a debate. Before I legalize it nationally, I want to make sure we know a lot more about the science behind it," Biden said.

He said it "is not irrational to do more scientific investigation to determine, which we have not done significantly enough, whether or not there are any things that relate to whether it's a gateway drug or not."

The National Institute on Drug Abuse agrees with Biden that more study is needed. It said research shows that most users of hard drugs tried marijuana first, but the majority of people who use marijuana don't go on to more dangerous substances. It said another explanation could be that "people who are more vulnerable to drug-taking are simply more likely to start with readily available substances such as marijuana, tobacco or alcohol."

Biden's stance contrasts with most other Democratic candidates who seek the nomination from voters who heavily favor legalization. According to a poll from the Pew Research Center that was released Thursday, 78% of Democrats and Democratic-leaning independents say marijuana should be legal.

Presidential pot: Why 2020 Democrats are ready to legalize marijuana

Sen. Bernie Sanders of Vermont vowed to legalize marijuana by executive order in his first 100 days if he is elected president. Sanders is a co-sponsor of a bill, along with fellow 2020 candidates Sens. Elizabeth Warren, Kamala Harris, Cory Booker and Michael Bennet, that would make marijuana legal and expunge criminal records.

South Bend, Indiana, Mayor Pete Buttigieg, who acknowledged last month that he used pot a "handful of times a long time ago," favors total legalization and the expungement of records. He opposes incarceration for the possession of any drug.

Eleven states and the District of Columbia have approved the use of recreational marijuana, and 33 states allow the use of medical marijuana. Fifteen states have decriminalized marijuana possession.

Contributing: Trevor Hughes, USA TODAY; The Associated Press

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Joe Biden through the years