In landmark action Wednesday, the General Assembly put Virginia on track to legalize possession of a small amount of marijuana on July 1.

Democratic majorities in the state House and Senate approved amendments that Gov. Ralph S. Northam sent down to speed up the legalization that was set to become effective three years from now in the original legislation that was sent to him.

The push to speed up this change became possible after Democratic Majority Leader, Delegate Charniele L. Herring of Alexandria, dropped her opposition to legalizing marijuana.

She wanted to wait until the state’s new marijuana regulatory agency is created, develops regulations and begins issuing licenses for the manufacture and retail sale. She had been a key sponsor of the legislation in the House of Delegates.

Republicans did their best to block legalization, forming an almost unmovable bloc, despite polls suggesting 68 percent of Virginians support giving marijuana use for adults the same status as alcohol enjoys. In contrast, the 23-member Virginia Legislative Black Caucus provided a solid bloc of support.

In the 100-member House of Delegates, the final tally was 53- 44, with two members not voting. In the 40-member Senate, 20 Democrats voted yes and the 20 Republicans voted no. Democratic Lt. Gov. Justin E. Fairfax broke the tie by voting in support of the amendments to legalize this summer, though retail sales still will be banned.

Passage was tied to criminal justice reform in the wake of findings that Black people were four times more likely to face arrest and conviction for possession of marijuana than white individuals, even though use is about equal across racial lines.

The amended legislation, which Gov. Northam created after consultation with Democratic lawmakers, would allow anyone 21 and older to possess up to one ounce, or about 29 grams, of marijuana. According to estimates, that is enough to create 28 joints. The law also would allow people to grow up to four plants. Possession would be decriminalized for those under 21, with a first offense carrying a $25 fine.

Those adults still locked up for simple possession of marijuana would be able to seek immedi- ate release. People whose lives have been marred by arrests and convictions will be able to seek expungement of such charges from their criminal histories.

According to Gov. Northam, Virginia will be the 16th state and the first in the South to legalize marijuana.

Virginia decriminalized possession of small amounts of marijuana last year, making it a civil offense that carries a small fine.

Law enforcement organizations were among those urging the legislature to reject legalization, despite the enforcement disparity.

That disparity was a key reason the Richmond City Council on Monday backed a resolution proposed by 9th District Councilman Michael J. Jones urging passage of the governor’s amendments.

Still, Dr. James Avery said the rush to legalize ignores the ill effects that increased marijuana use will cause. An addiction expert and visiting professor at the University of Virginia, Dr. Avery said the modern varieties and growing methods make the marijuana now available far more potent and potentially addictive.

“This is not your father’s marijuana,” Dr. Avery said in an interview. “This is a far more potent drug.”

He said that the levels of THC, which creates the high, are 30 percent or higher, at least 15 times the levels that marijuana users experienced in the 1960s and 1970s when marijuana gained far wider popularity. “A hit or two is enough to create a high,” he said.

He predicted “an increase in the number of first-time users, particularly teen users, with greater prospects for addiction.” He said that studies indicate that with regular use, one in 11 adults and one in 16 teens becomes addicted.

Gov. Northam said the state is planning a marketing campaign to educate the public on the health and safety risk.