TRENTON - In a Capitol where Democrats rule both legislative chambers, it's not unusual to see Senate President Stephen Sweeney, whose South Jersey district trends more conservative than some of its North Jersey counterparts, clash with the more left-leaning Assembly leadership.

Yet political observers say they're stumped by Sweeney's opposition to a gun-control bill most of the state Democratic Party supports.

Sweeney is refusing to let the Senate vote on a bill that would decrease magazine capacity limit in New Jersey to 10 bullets from the current 15. The Assembly passed a bill in February lowering the limit, but it wasn't among the dozen gun-control bills heard in Senate committee last week.

"It's very odd. This is something that the entire Democratic Party is behind," said Patrick Murray, a political analyst at Monmouth University. "The only one who benefits from Sweeney burying this bill is Gov. Christie."

Christie, a potential Republican presidential contender in 2016, has said the 15-round limit is sufficient. Six states and Washington, D.C., have lower limits. Christie, who is running for reelection in November, has suggested that lawmakers focus on better access to mental-health treatment and stiffer penalties for gun traffickers and straw purchasers, elements included in their packages. All 120 legislative seats also are on the fall ballot.

Sweeney has largely echoed the governor's call for a focus on illegal guns. He also wants to overhaul the state firearms identification system, encoding firearm-owner information on driver's licenses or other state-issued ID. The 40-page bill was among the dozen favorably moved out of the Senate Law and Public Safety Committee.

But the Assembly last week doubled down on the magazine bill, bringing families of the children slain in the Sandy Hook Elementary School shooting to the Statehouse for a news conference in support of the bill. In an unusually public rift, Assembly Majority Leader Louis Greenwald (D., Camden) said he would not support a gun-control package that doesn't include the magazine limit.

Sweeney needs the Assembly's vote to pass his ID bill and other gun-control measures introduced in the Senate.

"If this tragedy had been transplanted from Connecticut to New Jersey, the reality is we'd be passing this legislation now," Greenwald said of the Sandy Hook school shooting that killed 20 children, four teachers, and two administrators.

Gun-control advocates say lower-capacity magazines could save lives in a mass shooting because the shooter would have to stop to reload. Eleven Sandy Hook children were able to escape a classroom when the shooter, Adam Lanza, paused to reload a 30-round magazine. Jared Lee Loughner, who killed six people and severely wounded U.S. Rep. Gabrielle Giffords in January 2011 in Arizona, was tackled when he stopped to reload his gun; he used 33-round magazines.

Bryan Miller, executive director of Heeding God's Call, an antigun-violence organization, said he was surprised to encounter resistance in a state like New Jersey.

"We're not talking about opposition that is the reason that this bill is not up. It could be up," he said at a news conference last Tuesday in Trenton. "It's up to the leadership. . . . It's time for the leadership to move it." Miller was a longtime leader of CeaseFire New Jersey, an organization that lobbied against gun violence.

Sweeney said the 15-round limit, the law in New Jersey for 20 years, "has been effective."

"What we must focus on now is preventing guns from getting into the hands of those who should not have them," he said in a statement. He declined further comment.

If Sweeney's resistance to the magazine bill is an attempt to appease gun owners in his district - which includes rural areas of Gloucester, Salem and Cumberland Counties - it might not work.

Frank J. Fiamingo, president of the New Jersey Second Amendment Society, said Sweeney had angered gun owners by supporting other gun-control bills.

"He told us, 'As long as I'm Senate president, there will be no further gun-control laws,' and he kept that promise for the longest time," Fiamingo said.

The magazine limit is a particularly sensitive topic with gun owners. They argue that criminals won't limit themselves to a 15-round magazine to follow the law, so why should a legal gun owner have fewer shots to fire during a break-in or a mugging?

Sweeney explicitly told him that the 10-round bill would go nowhere, Fiamingo said. To go back on that promise would be "a tremendous betrayal."

"He's not going to get the vote of the firearms owners like he has in the past, but we're not going to actively campaign against him," Fiamingo said.

But pass the lower magazine capacity bill, he warned, and "they'll eviscerate him. There will be nothing left but shredded underwear."

Murray disagrees.

"This one bill is not going to make or break Sweeney in the district," he said. Sweeney won his reelection bid in 2011 with 55 percent of the vote.

A majority of New Jersey voters support tightening federal gun-control laws, including mandatory background checks, a ban on assault weapons, and barring large-capacity magazines, according to a Quinnipiac University poll conducted in January.

But the poll did not define "high-capacity" when interviewing respondents, so it's unclear whether the 63 percent of South Jerseyans (those living in Burlington, Camden, Cape May, Cumberland, Gloucester, and Salem Counties) who expressed support for the measure would back a 10-round limit.

When asked whether New Jersey's gun laws were strict enough, 46 percent of South Jersey respondents said they should be stricter, 11 percent said they should be less strict, and 34 percent said gun laws should stay as they are.

Ben Dworkin, director of the Rebovich Institute for New Jersey Politics at Rider University, predicted the bickering among Democrats would not last long. Bringing a gun-control package to Christie, who will have to answer to New Jersey voters this year and, perhaps, to Republican primary voters down the road, is a maneuver Democrats can't pass up.

"It's a huge political opportunity to put the governor in a tight box," he said. In a Republican presidential primary, "he is going to be challenged by the folks who are much more absolute on this issue."