(CNN) Joe Biden said Saturday he still opposes legalizing marijuana on the federal level, telling attendees at a town hall in Las Vegas that he wants to see more study on whether it is a "gateway drug."

The former vice president's position puts him at odds with the other leading contenders for the 2020 Democratic presidential nomination, with the rest of the field calling for the federal government to legalize marijuana -- and several candidates criticizing Biden's position on Sunday and Monday.

Polls have shown that marijuana legalization is broadly popular with Democratic primary voters and the general electorate, and Biden's rivals, including Vermont Sen. Bernie Sanders and Massachusetts Sen. Elizabeth Warren, have discussed the issue as a matter of racial justice.

Biden, who has long opposed marijuana legalization, was asked Saturday if his position had changed.

"But let me tell you 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," he said. "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."

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