The 2020 Democratic hopeful who may be most vulnerable on marijuana policy is former Vice President Joseph R. Biden Jr., who in the 1980s and 90s was a leader in the “war on drugs.’’ He championed laws that set tough mandatory minimum sentences for drug offenses, including possessing large amounts of marijuana. This led to an era of mass incarceration, with a lasting economic and social toll for minorities.

“In 1994 Biden had a fairly mainstream position, but in 2020 that position is so far from the mainstream of Democratic politics that it is almost offensive,” said John Hudak, a senior fellow at the Brookings Institution who focuses on state and federal marijuana policy. He called Mr. Biden’s history on criminal justice issues “his biggest liability in the 2020 primary.”

Mr. Biden has apologized for parts of his record, calling a law from the 1980s requiring harsher penalties for crack cocaine than powder cocaine — the former was more popular with African-Americans and the latter with whites — “a big mistake.”