SAN FRANCISCO — California, Massachusetts and Nevada legalized medical marijuana on Tuesday in what advocates said was a reflection of the country’s changing attitude toward the drug.
Leading up to the election, recreational marijuana use was legal in four states: Alaska, Colorado, Oregon and Washington, along with Washington, D.C.
With the addition of California, Massachusetts and Nevada, the percentage of Americans living in states where medical marijuana use is legal for adults rose above 20 percent, from 5 percent.
Representative Earl Blumenauer, Democrat of Oregon and a supporter of legalization, said Tuesday’s votes would add to the pressure on the federal government to treat cannabis like alcohol, allowing each state to decide on its own regulations.
“The new administration is not going to want to continue this toxic and nonproductive war on drugs,” Mr. Blumenauer said.
The federal government’s ban on the drug precludes the interstate sale of cannabis, even among the states that have approved its use. But Tuesday’s votes created a marijuana bloc stretching down the West Coast, and Gavin Newsom, the lieutenant governor of California, said he saw an opportunity for the states where recreational marijuana is now legal to “coordinate and collaborate” on the issue, including applying pressure in Washington to relax the federal ban.
A Gallup poll in October found nationwide support for legalization at 60 percent, the highest level in the 47 years the organization has tracked the issue.
Support is rising even though some public health experts warn that there have been insufficient studies of the drug’s effects and that law enforcement agencies lack reliable tests and protocols to determine whether a driver is impaired by marijuana.
Supporters in California portrayed legalization as both a social justice and a criminal justice issue, saying the measure would help redress the disproportionate numbers of arrests and convictions among minorities for drug crimes.
“I think of this victory in California as a major victory,” said Lauren Mendelsohn, the chairwoman of the board of directors of Students for Sensible Drug Policy, a group that has campaigned against the government’s war on drugs. “It shows the whole country that prohibition is not the answer to the marijuana question.”
Ms. Mendelsohn spoke at a celebration in Oakland for the passage of Proposition 64, as California’s legalization measure was known.
Supporters of legalization in California vastly outspent opponents.
As of Nov. 6, pro-legalization committees in the state had raised around $23 million, according to the California secretary of state’s office. Chief among the backers were marijuana companies and tech entrepreneurs, including Sean Parker, a founder of the file-sharing service Napster and a former president of Facebook, who was the single largest donor to the campaign. The anti-legalization campaign had spent less than $2 million in California.
Kevin Sabet, the president of Smart Approaches to Marijuana, one of the country’s major funders against marijuana legalization initiatives, attributed the imbalance in campaign spending to investments by marijuana companies hoping to profit if the industry was legalized.
“There’s a lot of money to be made if marijuana is legal, not a lot of money to be made if it remains illegal,” he said.
Opponents of legalization say the adoption of medical marijuana laws in more than 25 states has led to a popular perception that cannabis is good for you. They have called for more studies on the drug’s long-term effects, particularly on the developing brains of young people.
“There is likely medical promise in the marijuana plant, but that is different than saying smoked marijuana is medicine,” Mr. Sabet said. “We wouldn’t smoke the opium plant to get the beneficial effects of morphine.”
A bill to legalize marijuana in Vermont, supported by Gov. Peter Shumlin, a Democrat, failed earlier this year. But in Massachusetts, public support for legalization rose during the fall, even with bipartisan opposition from the state’s top elected officials — including Gov. Charlie Baker, a Republican, and Attorney General Maura Healey, a Democrat — and an organized anti-legalization campaign.
Lawmakers in Rhode Island were watching Massachusetts closely, and they are expected to take up a legalization measure of their own now that one has passed there.
Two other states — Arizona and Maine — were voting on recreational marijuana legalization Tuesday. In Maine, a state with a libertarian streak that began decriminalizing marijuana decades ago, the referendum on legalization drew scant funded opposition.
Still, proponents of legalization said California would represent the biggest victory because of its huge economy and population and also its fertile soil and amenable climate.
Tuesday’s vote reinforced the state’s position as the epicenter of marijuana cultivation for the country, a role it has had illicitly for decades. Marijuana companies have been positioning themselves for the prospect of interstate commerce, buying large plots of land in areas that now grow vegetables and other crops.
The California measure allows people over 21 to possess limited amounts of marijuana for personal use and also permits the personal cultivation of up to six plants in private residences, provided they are shielded from public view. The sale of recreational marijuana will not be allowed until licenses are issued, a process that will take at least two years, said Steve DeAngelo, the founder of Harborside, a medical marijuana dispensary in Oakland.
California officials expect additional tax revenue of around $1 billion from marijuana sales. The revenue is earmarked for the study of medical marijuana, for the California Highway Patrol to develop procedures to determine driver impairment due to marijuana consumption, for youth education on drugs, and for the prevention of environmental damage from marijuana production, among other programs.
Support for legalization in California cut across all age groups except voters over 65, according to a Field poll released on Friday. Among those older voters, 42 percent were in favor, and 57 percent were against.
A large majority of Republicans in the poll, 65 percent, were against the measure, compared with 72 percent support among Democrats.
Support has been rising steadily since the 1960s, when only around 10 percent of California adults favored legalization, according to a 1969 Field poll, and legalization was the culmination of decades of campaigning by proponents. A measure to decriminalize marijuana in 1972 was soundly rejected in California, with 66.5 percent of voters opposed to it. In 1996, California voted to allow medical marijuana. But a 2010 measure to permit recreational use failed.
In addition to Tuesday’s votes on recreational marijuana, Arkansas, Florida, Montana and North Dakota all had medical marijuana initiatives on the ballot.