But Hickenlooper, the governor at the time of the 2012 initiative allowing recreational use of cannabis, eventually changed his mind.

He acknowledged that fears of increased use by children did not materialize, and he boasted of the tax revenues for social programs that regulated sales delivered.

Entering the Democratic presidential race this month, Hickenlooper joined a field already jammed with pro-legalization candidates, a reflection of swiftly changing public opinion since Colorado became the first of 10 states with legal recreational marijuana.

The issue today is a pillar of progressive politics, but not because of graying hippies who like their Rocky Mountain High. Rather, for many Democrats, legalization has become a litmus test for candidates’ commitment to equal treatment for all races in policing and criminal justice as well as fighting economic inequality.

“A Democrat who is not on board with legalization or addressing it in terms of repairing harms brought by prohibition for decades is going to have a tough time convincing any voter they’re serious about racial justice,” said Vincent M. Southerland, executive director of the Center on Race, Inequality and the Law at New York University Law School.

Sen. Cory Booker, D-N.J., last month introduced the pointedly named Marijuana Justice Act, which would legalize the drug nationwide and expunge past convictions. Supporters note that African-Americans are almost four times more likely to be arrested for marijuana possession than whites, even though rates of use are similar.

“It’s not enough to legalize marijuana at the federal level — we should also help those who have suffered due to its prohibition,” Booker said in a tweet.

Other 2020 candidates in the Senate quickly signed on as sponsors, including Kirsten Gillibrand, D-N.Y.; Bernie Sanders, I-Vt.; Elizabeth Warren, D-Mass.; and Kamala Harris, D-Calif.

Beto O’Rourke of Texas, who often hedges when asked about contentious policy issues, has endorsed the federal legalization of marijuana unequivocally. Asked about it Thursday — at his first event as a presidential candidate, in Keokuk, Iowa — O’Rourke framed his position as a matter of practicality and racial justice, given that people of color are imprisoned at disproportionate rates for drug offenses.

“I say this as the father of a middle school student, where middle schools are one of the fastest growing markets for marijuana sales today,” he told a coffeehouse crowd. “In the black market, they do not ID — they do not care — as long as they can make that sale.”

The 2020 Democratic hopeful who may be most vulnerable on marijuana policy is former Vice President Joe Biden, 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 Biden’s history on criminal justice issues “his biggest liability in the 2020 primary.”

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.”

Spokesmen for Biden, who has told aides he is 95 percent committed to getting into the presidential race, did not respond to requests for comment.

Two Democratic senators opposed to legalizing marijuana at the federal level — Amy Klobuchar of Minnesota, who entered the race last month, and Sherrod Brown of Ohio, who opted out of running this month — represent battleground states in the Midwest, whose politics are less progressive than the coastal states that led the way to legalizing cannabis for adults.

Klobuchar, who hopes to appeal to moderate voters, has said she supports the right of states to decide individually about legalization, rather than the approach of Booker, O’Rourke and others who want to shift policy at the national level.

In 2015, Ohio voters overwhelmingly defeated a referendum to legalize recreational marijuana.

Brown told an Ohio TV station last year that the jury was out on the experience of states that had legalized the drug, including whether it was a “gateway” to harder drugs.

Midwestern states have been devastated by opioid overdoses. Marijuana has been shown to lead in some cases to increased alcohol abuse, but the majority of users do not progress to harder drugs, according to the National Institute on Drug Abuse. A study last year in the journal JAMA Internal Medicine found that after states legalized marijuana for medical use, prescriptions for opioids dropped nearly 15 percent.

Polling, which has found a rapid increase in support for legal recreational marijuana since 2000, including majorities in both parties, shows little regional variation. Support in the Midwest and South is about equal to the coasts. Legalization is most popular with young adults under 35, with nearly 4 out of 5 in favor — catnip to Democrats seeking to rally millennial voters, whose disapproval of President Donald Trump is higher than any other age group.

Experts said opposition is concentrated among social conservatives.

The racial demographics of support have also shifted from a time when whites favored adult recreational use and minority groups opposed it, said Jonathan P. Caulkins, a professor at Carnegie Mellon University, who has tracked marijuana policy for more than a decade. “Over the last 15 to 20 years, African-Americans have switched to favor legalization,” he said.

Harris, a former attorney general of California, who is black, exemplified the change. In 2014, she said it would be irresponsible for her as the state’s “top cop” to support California’s legalization of recreational cannabis, which passed in a referendum two years later.

By 2017, Harris declared the war on drugs a failure and called it “a war on poor communities more than anything.” In an interview last month, when pressed on whether she was anti-marijuana, she said she had smoked it in college. “Half my family’s from Jamaica,” she joked. “Are you kidding me?” (Her Jamaica-born father, Donald J. Harris, was not amused and called this a “fraudulent stereotype.”)

In last year’s midterm elections, Michigan voters made the state the first in the Midwest to legalize recreational marijuana. Meanwhile, Missouri, Oklahoma and Utah overwhelmingly approved medical marijuana, adding to a total of 33 states and the District of Columbia to do so. Democratic gains in state government are pushing several toward legalization this year, including New York and New Jersey.

Trump has been inconsistent on the issue, saying that legalization “should be up to the states, absolutely” during the 2016 campaign, but appointing Jeff Sessions as attorney general, who cleared the way to crack down on states that had legalized the drug.

Last year, Trump signaled support for a bipartisan bill introduced by Sens. Cory Gardner, R-Colo., and Warren to shield state-licensed cannabis businesses from federal prosecution.

The bill highlights that marijuana is not strictly a left-right issue, as does Biden’s history of supporting the drug crackdown.

Whether primary voters penalize Biden over his past remains an open question.

A recent poll of likely Democratic primary voters in New Hampshire by the Democratic firm Change Research seemed to suggest he may be putting the issue behind him. It found that even among those who called marijuana legalization their No. 1 priority, Biden was still the second-most popular candidate, after Sanders.