"When you meet families that lost their loved ones, it's pretty hard to explain why you can't do a simple thing like this to make lives a little bit easier," said Senate President Stephen Sweeney (D., Gloucester).
The Assembly passed the same bill last year, but Sweeney refused to bring it up for a vote in the Senate. On Monday, he said he changed his position after speaking with people like the Sandy Hook parents. "And, honestly, I'm proud I did," he said.
"To lawful gun owners - who I have great deal of respect for - we're not hurting one of them," said Sweeney, who grew emotional at the news conference. "For sportsmen, we're not hurting one. And if it prevents one child or one person from losing their life, we should do it."
The proposal comes even as gun-control legislation stalled in Washington last year despite a high-profile push by President Obama to renew an assault-weapons ban and mandate background checks for all gun sales.
Gun-rights groups have pledged to fight New Jersey's measure, calling it a failed policy and an infringement on the Second Amendment right to bear arms.
"This legislation represents the largest gun ban and gun-confiscation scheme in the history of the state. It will make felons out of potentially hundreds of thousands of law-abiding citizens overnight," said Darin Goens, state liaison for New Jersey of the National Rifle Association's lobbying arm.
Gov. Christie, a Republican, will review the bill and "make a decision in due course" if it reaches his desk, a spokesman said.
"New Jersey already has the third-toughest gun laws in the entire country and is one of only seven states that has an existing law on the books limiting the size of ammunition magazines," spokesman Colin Reed said in an e-mail Monday, pointing to a 2013 list compiled by the Law Center to Prevent Gun Violence and the Brady Campaign to Prevent Gun Violence.
Among other laws, the state prohibits possession of assault weapons and limits the number of handguns that can be purchased to one a month.
When Congress passed an assault-weapons ban in 1994, it restricted magazine capacity to 10 rounds. The law expired in 2004.
Six states and the District of Columbia restrict magazine capacity to 10 rounds, according to the Law Center to Prevent Gun Violence.
Gun-control advocates say reducing magazine capacity increases the number of times a shooter has to reload, improving the odds of escape or the possibility of intervention. At Sandy Hook, the advocates said, 11 children escaped while 20-year-old Adam Lanza reloaded his gun.
Lanza, who fatally shot 20 schoolchildren and six adults, was armed with high-capacity 30-round magazines. Gunmen also used high-capacity magazines in shootings in Aurora, Colo.; Tucson, Ariz.; and Virginia Tech, according to the Law Center to Prevent Gun Violence.
"I have three children. James is 13. Natalie is 12. And our sweet little Daniel was 7 when he was murdered in his first-grade classroom at Sandy Hook Elementary," Mark Barden, advocacy director of Sandy Hook Promise, said at the news conference.
Holding up a picture of Daniel, he said: "We talk about policy and solutions and all of that - I think it's important to keep in mind the face, the personal aspect of what's at stake here."
His group has traveled to statehouses in Pennsylvania, Delaware, Ohio, Connecticut, and elsewhere in the last year to promote gun safety.
"This is about toppling a norm within this country," said Nicole Hockley, also of Sandy Hook Promise, whose 6-year-old son, Dylan, was killed. "To be honest, who can accept the fact that it is normal, the amount of school shootings, the amount of shootings every day?"
Assembly Minority Leader Jon Bramnick (R., Union) said he met with the Sandy Hook parents Monday and would give them the opportunity to "make their case to members of the Republican caucus."
"Obviously, when you meet with parents who have lost their children, it's a very difficult and emotional meeting," Bramnick said. "But as legislators, our job is to look at the big picture and determine the best way to go."
Sandy Hook families came to Trenton last year in support of other gun-safety laws.
Gov. Christie signed a package of bills that required certain mental-health records to be sent to a national background check database; increased penalties for gun trafficking; and disqualified anyone on the FBI's Terrorist Watch List from being able to obtain a gun permit or a firearms identification card.
But he vetoed legislation that would have banned the sale of Barrett .50-caliber rifles, which are used for long-range, precision shooting, even though he initially supported the idea. He said Democrats had gone too far in their proposal by applying the ban retroactively to those who already legally owned the gun.
When Christie was reviewing some of the proposals last year, a gun-rights group in New Hampshire, an early presidential primary state, asked its supporters to lobby against those measures by telling Christie they were "watching with 2016 in mind."