Sen. Dianne Feinstein, D-Calif., and Rep. Carolyn McCarthy, D-N.Y., unveiled legislation Thursday that would reinstate and considerably expand the expired federal ban on assault weapons and large-capacity ammunition magazines.

Opposition from Republicans, the National Rifle Association and potentially some Democrats, however, spells trouble for the measure, which is a central component of President Barack Obama’s plan to address gun violence after the fatal shooting of 26 people at a Connecticut elementary school on Dec. 14.

The two chief sponsors outlined the legislation during a crowded news conference on Capitol Hill, surrounded by law enforcement officers, faith leaders, mayors, survivors of gun violence and other Democratic supporters of the bill, including Senate Majority Whip Richard J. Durbin of Illinois and Conference Vice Chairman Charles E. Schumer of New York.

Feinstein said the bill, which she has worked on for more than a year, is a legislative response to mass killings such as the one that took place in Newtown, Conn.

“The common thread in these shootings is that each gunman uses a semi-automatic assault weapon or a large-capacity ammunition magazine,” Feinstein said. She said she was introducing the Senate legislation on Thursday; McCarthy will introduce her bill once the House is back in session, an aide said.

The bill would ban the future sale, transfer, manufacture and importation of 157 specific kinds of semi-automatic guns and impose the same restrictions on ammunition magazines that contain more than 10 rounds. It would also ban rifles, handguns and shotguns that accept detachable magazines and have certain physical characteristics, including a pistol grip or folding stock.

Feinstein said the legislation would simplify the definition of an assault weapon by reducing the number of defining physical characteristics from two to one, making it more difficult for gun manufacturers to design firearms that get around the ban. Unlike the previous assault weapons ban, which Feinstein wrote and was in effect from 1994 to 2004, the new version does not have a sunset date and is intended to stay on the books permanently.

The proposal includes some provisions that represent a significant expansion of the earlier ban and are likely to fan the fears of gun owners that their Second Amendment rights are being compromised.

Most prominently, the bill addresses the millions of semi-automatic guns and large-capacity ammunition magazines that are already in private owners’ possession today. It would require background checks on all such firearms if they are sold or transferred— including from one private citizen to another — and it would ban the future sale of large-capacity magazines even if those magazines are currently in their owners’ hands legally. It would impose a “safe storage requirement” for firearms currently in existence, and it includes a voluntary gun-buyback program designed to encourage gun owners to turn over their firearms in exchange for money provided through a Justice Department grant program.

“We have learned from the [original] bill,” Feinstein said.

Lukewarm Democratic Support

The legislation is seen as the toughest of the president’s gun control measures for proponents to try to pass in a politically divided Congress, and there were immediate signs Thursday of those challenges.

Hours after Feinstein and McCarthy unveiled the proposal, Sen. Ted Cruz, R-Texas, issued a statement saying he looks “forward to helping lead the fight to defeat this bill.”

“Washington politicians shouldn’t be taking advantage of recent tragedy to try to push an aggressive gun control agenda,” Cruz said. “This proposal would have done nothing to prevent the terrible murders in Newtown, but it would limit the constitutional liberties of law-abiding citizens.”

Cruz’s comments came a day after Speaker John A. Boehner, R-Ohio, said during an event at the Ripon Society, a Republican think tank, that an assault weapons ban would not be an effective response to gun violence and would “give people a false sense of security.”

Democrats from gun-friendly states also took a cautious approach Thursday, which suggests that the measure will have enough problems finding sufficient votes in the Senate, let alone in the House.

“I’m not ready to sign up for anything,” Sen. Claire McCaskill, D-Mo., said.

Sen. Max Baucus, D-Mont., who has a top rating from the NRA and faces re-election in 2014, added: “I’m going to listen to Montanans and make my decision.”

Senate Majority Leader Harry Reid, D-Nev., who did not attend Thursday’s news conference, has shown only tepid support for the assault weapons proposal. He told a Nevada television station Jan. 11 that he sees little chance of the measure becoming law due to likely opposition in the House. Reid may choose not to force some of his members into a difficult vote if the legislation stands no chance of passing the House.

Influence Battle

The NRA has vowed to fight Obama’s gun initiatives forcefully, and it released a survey Thursday refuting assertions by Democrats that many NRA members would support steps such as an assault weapons ban.

The survey the group issued found that 89 percent of NRA members “oppose banning semi-automatic firearms,” which it said are wrongly characterized as “assault rifles.”

Several speakers at Feinstein’s news conference acknowledged that a bruising lobbying battle with the NRA is likely, but they expressed confidence that they could outmaneuver pro-gun rights advocates by appealing directly to Americans who have seen too many mass shootings.

“I believe the gun lobby is no match for the cross lobby,” said the Rev. Gary Hall, dean of the Washington National Cathedral.

Democratic supporters of the legislation cast themselves as supporters of the Second Amendment and said they are not seeking to take guns away from law-abiding citizens, particularly hunters and sportsmen.

Feinstein’s bill specifically allows 2,258 “legitimate hunting and sporting rifles and shotguns” as well as “any gun manually operated by bolt, pump, lever or slide action,” according to background information provided by her office. Guns carried by government officials and law enforcement also would be exempt from the ban.

“None of us want to take away the hunting rifle that your Uncle Tommy gave you when you were 14 years old,” Schumer said.

Legal Questions

Renewing the assault weapons ban is not just a question of political will but also, potentially, of the proposal’s legal viability.

The Supreme Court ruled in 2008 that individuals have a Second Amendment right to the possession of firearms for self-defense, and some scholars believe the court’s decision has complicated the legal terrain for Feinstein’s legislation.

The court held, for example, that any ban on certain kinds of firearms would need to show that those guns are “dangerous and unusual.” Many of the guns that would be banned under Feinstein’s bill — as well as the ammunition magazines that feed them — are in widespread circulation and common use.

Gun control advocates counter that the court’s ruling in the 2008 case, District of Columbia v. Heller, applied only to the possession of handguns for self-defense purposes and does not prohibit reasonable government regulations on more dangerous kinds of firearms.

Schumer said Thursday that the proposed assault weapons ban is “certainly within the framework of the Heller decision.”