Adam Eidinger, co-owner of Capitol Hemp, is a leader in the push to legalize the possession and use of small amounts of marijuana in the District. (Photo by Michael S. Williamson/The Washington Post) (Reuters)

Adam Eidinger, co-owner of Capitol Hemp, is a leader in the push to legalize the possession and use of small amounts of marijuana in the District. (Photo by Michael S. Williamson/The Washington Post) (Reuters)

(Update: House Republican vows to upend D.C. ballot measure legalizing marijuana )

The District followed Colorado and Washington state into a closely watched experiment to legalize marijuana Tuesday, as voters overwhelmingly backed an initiative 7 to 3 allowing cannabis to be consumed and grown in the nation’s capital. The move to allow the drug almost certainly will take effect unless the next Congress blocks it.

Under a voter-proposed measure, known as Initiative 71, residents and visitors age 21 and older will be allowed to legally possess as much as two ounces of marijuana and to grow up to three marijuana plants at home. Leading candidates for mayor and the D.C. Council have vowed to quickly sign the measure into law. A majority of the council also pledged that if approved by voters, they would submit follow-up legislation to Congress next year establishing a system to sell and tax the drug in the District.

The twin measures will become law, as District bills do, unless Congress vetoes them and the president agrees that the local measures should be halted. That complex layer of federal oversight could thrust Congress — which on Tuesday flipped to Republican control — and President Obama into the middle of a rapidly evolving national debate.

In joining two states to bring marijuana into the mainstream — making it nearly akin to alcohol and tobacco — the District’s vote is the latest sign of growing public acceptance of the drug. Advocates have been trying to give marijuana legal status since the 1960s, losing periodic battles with parent groups and to the war on drugs. But the arguments against weed have lost steam, and public opinion has shifted; about 6 percent of Americans use the drug, including one-third of the nation’s high school seniors.

Unlike the arguments about health concerns elsewhere, the legalization debate in the District became fused with weighty issues of civil rights after a series of studies during the past year showed wide disparities in drug arrests: Eighty-eight percent of people convicted of marijuana possession in the city in recent years were black, even as surveys have shown that whites and blacks are equally likely to use the drug.

“The population in the District is certainly different from that in Colorado and Washington state. Here, this has been cast as a racial-justice issue,” said Malik Burnett, a doctor who delayed practicing to organize support for the measure. “This is huge. We’re talking about ending the prohibition of marijuana as a manifestation of the war on drugs, in the birthplace of the war on drugs: Washington, D.C.”

Adam Eidinger, who is a longtime advocate for legalization in the District and who spent $20,000 of his own money to help put the measure on the ballot, said he was thrilled and confident that any pushback from Congress could eventually be overcome.

“This sends a message to the nation that people are finally ready for change,” he said. “If your job is over at 5 o’clock and you want to have cannabis instead of a glass of scotch, so be it.”

Realtor Tom Bryant, 50, who cast a ballot at the Georgetown public library, said he voted yes because he thinks it is “ridiculous for people to go to jail” for a small amount of pot. Katie Holloran, a 37-year-old teacher, said she, too, voted in favor of legalization: “I guess I’ve really never understood why it’s different from something like alcohol.”

Not everyone agreed. Alizonia Leach, who voted Tuesday at Watkins Elementary School, said she had no doubts: “Oh, no, no, no, no, no, no,” she said. “Marijuana is not good for anybody.”

This summer, the District joined 17 states that have decriminalized marijuana. Members of the D.C. Council and Mayor Vincent C. Gray (D) said they were moved by studies that showed the District’s marijuana-arrest rate was higher than any of the 50 states and ranked seventh nationally among a study of 1,000 counties analyzed by the American Civil Liberties Union.

Such studies also helped fuel a complete reversal in public opinion during the past four years among the District’s black residents, who now account for half the city’s population.

According to Washington Post polls this year, roughly 56 percent of likely African American voters said they planned to back legalization. Four years ago, 37 percent were in favor and 55 percent opposed, with many saying they feared greater access could lead to addiction among black youths.

The D.C. Council measure in March that decriminalized marijuana took a first step toward legalization. It stripped away jail time for possession and made it a $25 fine — cheaper than most city parking tickets — and the lowest fine outside of Colorado, Washington state or Alaska. Penalties for public consumption also were lowered to that of carrying an open container of alcohol, punishable by up to 60 days in jail.

But on the District’s iconic federal land, including the Mall, the monuments and streets surrounding the White House, possession remains a federal offense punishable by up to a year in jail.

The District also is home to federal agencies charged with enforcing U.S. drug laws, which still designate marijuana in a class of the most dangerous drugs, worse than cocaine and viewed equal only to the likes of heroin in terms of how addictive they are.

Full legalization of marijuana would set up a conflict with federal law enforcement agencies and Congress. Even advocates of legalization say they can barely imagine a day when the District would resemble Denver, with a proliferation of shops selling marijuana by the bag, in joints or in foods such as cookies or brownies.

Advocates testified at a hearing last week that, given the necessary congressional review and time needed for the D.C. Council to decide how sales would work, the earliest marijuana could be legally purchased in the District would be in early 2016.

Less clear is how Congress will react. After D.C. voters passed a measure allowing medical marijuana dispensaries in 1998, Republicans used amendments to federal budget bills for 11 years to keep the District from enacting the law.