SAN FRANCISCO — Signaling a major gun-rights battle to come, Lt. Gov. Gavin Newsom and a group of activists announced their intention Thursday to put a tough firearms-control measure on California’s ballot next year aimed at requiring background checks for ammunition purchases and forcing gun owners to give up large-capacity magazines.

The move — coming after another round of mass shootings and Democratic presidential candidate Hillary Clinton’s new push for more national gun control — suggests the incendiary issue is moving again to the center of the political debate, after several years of politicians avoiding it.

But it carries risks for gun-control supporters like Newsom, who is running for governor in 2018, and also for fellow California Democrats running in swing districts in 2016, who could become vulnerable to a conservative backlash.

Newsom and gun-control activists announced the proposed voter initiative Thursday at a news conference in San Francisco’s Financial District, outside the 101 California Street office building where nine people were killed and six injured in a 1993 shooting rampage.

Gun-rights advocates on Thursday immediately swung into action, vowing to “bring the fight” to Newsom’s political doorstep. But Newsom said he’s ready to rumble.

“If you’re going to do something, go big,” he said in an interview after the news conference.

“There’s no doubt this is emotional on all sides — for parents like myself, it’s understandably emotional,” said the father of three with a fourth on the way.

More American children die from gun violence each year than police officers killed in the line of duty, he said. “It’s insane.”

Newsom and gun-control groups will need signatures from 365,880 registered voters to place the initiative on the November 2016 ballot, which will likely be full of other controversial and costly measures, including proposals to legalize recreational marijuana, raise the tobacco tax and overturn the state’s ban on single-use plastic grocery bags.

California already has some of the nation’s toughest gun laws, including a 10-day waiting period for all firearm purchases, an assault weapons ban, and a ban on making and selling magazines that hold more than 10 rounds.

The state enacted its assault weapons ban in 1989 and expanded it 10 years later. But those who already owned the banned guns and magazines were allowed to register and keep them.

Newsom’s measure would require owners to turn the outlawed magazines into police for destruction, sell them to a licensed firearms dealer or move them out of the state — just as San Francisco supervisors and Sunnyvale voters chose to require in 2013. New York, New Jersey, Hawaii and the District of Columbia also have such laws.

“The large-capacity magazine ban is going to stimulate a lot of opposition; that’s going to hit a lot of ordinary gun owners where it hurts” — including some who might be open-minded to other kinds of gun control, said Adam Winkler, a UCLA law professor and author of 2011’s “Gunfight: The Battle over the Right to Bear Arms in America.”

“It plays into the hands of gun-rights proponents who are always warning that the government is going to come take your guns,” Winkler said.

Newsom’s measure also would equire licensing of ammunition sellers and instantaneous point-of-sale background checks for all ammunition purchases to weed out those convicted of a felony or a violent misdemeanor, those with restraining orders against them or those declared dangerously mentally ill.

No other state requires background checks for ammunition purchases.

The measure would also require firearm owners to notify law enforcement if their firearm has been lost or stolen. Eleven states and the city of Sacramento already require this, but Gov. Jerry Brown vetoed bills to do just that in 2012 and 2013.

“It’s a cynical shot by Gavin to push himself into the limelight in preparation for his run for governor,” said Eric Meyers, of Napa, president of the Liberal Gun Club’s California chapter, arguing that a Democrat truly serious about further reducing gun violence should propose job programs, health care access and better schools for inner cities.

Gun control can draw single-issue voters on both sides to the polls. For example, some political analysts believe that Democrat Tom Bradley lost 1982’s gubernatorial election to Republican George Deukmejian because of his endorsement of Proposition 15, which would have imposed stricter handgun controls. The measure sparked a huge backlash and increased voter turnout in the state’s more conservative areas.

But the attitudes of Californians might be different now because of the nation’s spurt of mass shootings. A Public Policy Institute of California survey released last month found 65 percent of California adults believe laws on gun sales should be stricter, well above the national rate of 52 percent. The poll found 82 percent of Democrats favor stricter laws, compared with 54 percent of independents and 36 percent of Republicans.

But Newsom’s initiative could cause conservatives to turn out disproportionately.

“Single-issue, pro-gun voters don’t usually have a great need to turn out for the presidential election — they know which way California is going to go,” Winkler said, but this measure “could stimulate more turnout among people who otherwise might skip this election altogether.”

That, Newsom acknowledged, “is a potential risk.” But “if you believe in something and think it’s the right thing to do, I think you’ve got an obligation to do it.”

Meanwhile, the grass-roots Firearms Policy Coalition on Thursday bought the web domain “stopNewsom.com,” which redirects to a page on the coalition’s site where people can sign up to donate or volunteer to fight Newsom’s plan or support a counterproposal protecting gun rights.

“If Gavin Newsom wants to declare war on law-abiding gun owners and Second Amendment rights, we’re certainly going to bring the fight to him,” said coalition president Brandon Combs, of Roseville.

Josh Richman covers politics. Follow him at Twitter.com/Josh_Richman. Read the Political Blotter at IBAbuzz.com/politics.