SACRAMENTO, Calif. — With the anniversary of last year’s terrorist shootings in San Bernardino approaching, California voters are considering expanding some of the nation’s toughest gun control measures through an initiative that would outlaw possession of large-capacity ammunition magazines and require permits to buy ammunition.

Proposition 63 also would extend California’s unique program that allows authorities to seize firearms from owners who bought guns legally but are no longer allowed to own them.

It would require offenders to give up their weapons as soon as they are convicted of a felony or a violent misdemeanor, found to be mentally unstable or are the subject of a restraining order involving domestic violence.

Its chief proponent, Lt. Gov. Gavin Newsom, touts that provision as a “game-changer” in the national debate over keeping firearms from those who are deemed dangerous.

The initiative set up a strange game of one-upmanship between Newsom and fellow Democratic state lawmakers who already approved variations of the ammunition background checks and large magazine ban this year.

That has made the debate unusually personal, with opponents accusing Newsom of furthering his political ambitions as he campaigns to succeed his fellow Democrat as governor in 2018. He in turn dismisses criticism even within his own party as pure politics, as when Senate President Pro Tem Kevin de Leon, D-Los Angeles, labeled the initiative “irrelevant” in July after lawmakers passed several competing measures.

But Gov. Jerry Brown vetoed several related bills, leaving those issues for voters to decide:

— The ballot measure would toughen the penalty for stealing a gun, reversing a portion of a previous initiative approved by voters two years ago that lowered some property crimes from felonies to misdemeanors.

— California would follow 11 other states in requiring that lost or stolen firearms be quickly reported to law enforcement. Brown said he thinks that provision would do little good because responsible owners already report their losses.

There is some overlap between the new gun laws passed by legislators and the initiative:

— The ballot measure would give prosecutors more discretion in punishing gun owners who don’t surrender large-capacity ammunition magazines. The already approved law will make continued possession of magazines holding more than 10 bullets an infraction similar to a traffic ticket. The initiative would let prosecutors charge the offense as either an infraction or a misdemeanor.

— The new law is intended to supersede the initiative on the way background checks would be conducted for those buying ammunition. But the initiative would eliminate some exceptions while also requiring background checks for ammunition dealers and requiring dealers to report lost or stolen bullets. Newsom says the courts will have to decide which takes precedence if the ballot measure passes.

Voters will consider the proposition nearly a year after the San Bernardino mass shooting by a radicalized husband and wife who killed 14 people before dying in a shootout with police.

“People in California have had enough of the gun murder rate. San Bernardino punctuated this,” Newsom said. Even if the proposed restrictions can’t stop mass shootings, they could help stem daily gun violence, he said, sending “a message that is going to ripple across the United States.”

Supporters have raised more than $5 million to advance the ballot measure, nearly 10 times as much as opponents, with the biggest donations coming from the California Democratic Party and Newsom’s campaign account.

The measure’s success in California will demonstrate that the National Rifle Association, though feared in Congress, is a “paper tiger” that can be defeated in the states, Newsom said.

“When Newsom wants to set up the NRA as the bogyman, what he’s really saying is all these law enforcement groups are stupid,” responded Chuck Michel, a spokesman for the opposition Coalition for Civil Liberties that includes the NRA.

Indeed, the California State Sheriffs’ Association and California Police Chiefs Association are among opponents.

“My chief concern is it will not achieve the stated goal of reducing violence and saving lives. Instead what it will do is negatively impact thousands and thousands of law-abiding citizens,” said Butte County Sheriff Kory Honea.

Ventura Police Chief Ken Corney, president of the chiefs’ association, said in an opposition letter that the chiefs supported much of this year’s gun control legislation, but fear the new laws would be undermined by the competing ballot measure that “fails to meet the appropriate balance between public safety and individual gun rights.”