More than two years after California voters made it illegal to possess high-capacity gun magazines commonly used in mass shootings, the National Rifle Association has tied the measure up in court. Now the gun lobby has set its sights on a nearly 2-decade-old law that banned the sale of those devices in the state.

That law, which took effect in 2000, has made it illegal to buy or sell magazines that can hold more than 10 cartridges. Proposition 63, sponsored by then-Lt. Gov. Gavin Newsom and approved by the voters in 2016, would require anyone who owns the magazines to turn them in, send them out of state or remake them to comply with the law.

If the NRA succeeds in killing one or both measures, they would be the organization’s first victories in a lengthy California campaign against some of the nation’s strictest gun-control laws. The final decision will likely rest with the U.S. Supreme Court and could signal whether President Trump’s new appointees have revived the court’s fervor on firearms issues after nearly a decade of silence.

The high-capacity magazines allow their owners to fire dozens of shots without reloading and have been used — along with semiautomatic rifles, some of which are banned in the state — in mass killings such as the school massacre in Newtown, Conn.

Tens of millions of Americans own firearms with those attachments, said Sean Brady, a lawyer for the NRA-affiliated California Rifle and Pistol Association. Every police department in the nation has them, too, he said, “to defend themselves and the public.”

California did not try to prevent owners of those magazines from keeping them until 63 percent of the voters passed Newsom’s Prop. 63 in November 2016.

One section of Prop. 63, a first-in-the-nation requirement of background checks before sales of ammunition, has taken effect while the NRA challenges it before a federal judge in San Diego. The other main provision, banning possession of high-capacity magazines, was due to take effect in July 2017 but was blocked two days earlier by the same judge, Roger Benitez, who issued a statewide injunction against it.

While state lawyers argued that the ban would save lives without interfering with Second Amendment rights of self-defense, Benitez said the new law “burdens the core of the Second Amendment by criminalizing the mere possession of these magazines that are commonly held by law-abiding citizens for defense of self, home, and state.”

And by requiring those citizens to sell them, modify them or send them out of state to avoid criminal penalties, the state is confiscating their property without compensation, Benitez said.

Attorney General Xavier Becerra appealed the ruling to the Ninth U.S. Circuit Court of Appeals, which has a history of upholding gun-control measures — including a ban on high-capacity magazines by the city of Sunnyvale that the court left intact in 2015.

But in July, a panel of the appeals court upheld Benitez in a 2-1 ruling that drew relatively little notice because it was not published as a precedent for future cases, although it maintained the statewide injunction.

Legal challenges to California gun laws by the NRA and its allies have become commonplace, particularly since the Supreme Court ruled in 2008 that the Second Amendment protects the right to possess guns at home for self-defense. Gun advocates have scored occasional victories before panels of the Ninth Circuit, such as a 2014 ruling striking down the state’s restrictions on carrying concealed handguns in public, but the court promptly granted the state attorney general’s request for a rehearing before a larger panel, which then upheld the law.

This time, though, Becerra’s office decided against seeking a rehearing, saying it wouldn’t resolve anything because Benitez was already considering arguments from both sides on whether to declare the voter-approved ban unconstitutional. The judge held a hearing in May, and has received further written arguments since then, but has given no indication when he will rule. And the NRA’s California affiliate has raised the stakes by also asking Benitez to overturn the 2000 law that banned sale or purchase of the magazines.

“The state’s total ban on magazines typically possessed for lawful purposes, including self-defense, plainly violates the Second Amendment,” lawyers for the association said in a filing with Benitez. “The availability of more ammunition in a firearm increases the likelihood of surviving a criminal attack.”

That’s not what the evidence shows in mass shootings, said Adam Skaggs, chief counsel for the Giffords Law Center to Prevent Gun Violence in San Francisco, which supports the state law.

“For reducing lethality and injury counts, this is the single most effective way to do that,” Skaggs said in an interview. Without a large-capacity magazine, he said, “you have to stop to reload, and can be disarmed. ... These are critically important laws. They don’t restrict anybody’s ability to use guns for self-defense.”

Laws similar to California’s have been upheld by courts throughout the country, Skaggs said, including a 10-cartridge limit in New Jersey that was affirmed by a federal appeals court last month. But in view of Benitez’s assessment of Prop. 63 in 2017, Skaggs said, no one should be surprised if he strikes down both the voter-approved ban on possessing large-capacity magazines and the 2000 legislative ban on buying or selling them.

The case would then return to the Ninth Circuit, whose record suggests that it would be likely to uphold both state laws, a process that could take a year or longer. The final decision would be up to the Supreme Court, which until recently had denied review of state laws regulating firearms, including the California concealed-handguns law, for nearly a decade.

On Tuesday, the court agreed to hear a challenge to a New York City law that prohibits residents from taking guns out of the home except to a firing range within the city. The case, to be heard in the term that starts in October, could allow the justices to examine an issue that has divided lower federal courts: whether the Second Amendment, interpreted in 2008 to allow in-home possession of handguns for self-defense, applies outside the home.

The court’s acceptance of the case may be an sign that Trump’s appointees, Justices Neil Gorsuch and Brett Kavanaugh, are more willing than their predecessors to take on firearms issues like the California gun-magazine laws. Kavanaugh, in particular, said as a federal appeals court judge in 2011 that the Second Amendment should be interpreted to allow possession of semiautomatic rifles, prohibit mandatory gun registration and allow only regulation that has been historically or traditionally accepted.

“It appears the current court probably will be more receptive to Second Amendment challenges,” said Skaggs, the Giffords Law Center attorney. In the meantime, he said, “it’s quite rare that something passed by a significant majority of California voters is held up by one judge.”

Newsom’s office did not respond to requests for comment.

Bob Egelko is a San Francisco Chronicle staff writer. Email: begelko@sfchronicle.com Twitter: @BobEgelko