Sen. Dianne Feinstein (D-Calif.) stands next to a display of assault weapons during a news conference Thursday on Capitol Hill in Washington, DC. (Alex Wong/Getty Images)

Democratic lawmakers formally reintroduced a bill Thursday that would ban military-style assault weapons and high-capacity ammunition magazines, the most ambitious — and politically risky — element of proposals unveiled by President Obama to limit gun violence.

The "Assault Weapons Ban of 2013" is a much more far-reaching proposal than the federal ban that expired in 2004. The proposal would ban the sale, transfer, manufacturing or importation of more than 150 specific firearms, including semiautomatic rifles or pistols that can be used with a detachable or fixed ammunition magazines that hold more than 10 rounds and have specific military-style features, including pistol grips, grenade launchers or rocket launchers.

It excludes more than 2,250 firearms used for hunting or other sport, and assault weapons lawfully owned before the law's enactment. But it would require background checks for the sale or transfer of grandfathered weapons and would bar the sale or transfer of large-capacity feeding devices owned before the bill's enactment. Current assault weapon owners also would need to safely store their firearms. Unlike the original federal ban passed in 1994, the new ban would be permanent.

The measure was unveiled Thursday morning by a slate of Democratic co-sponsors, led by longtime gun control advocates Sen. Dianne Feinstein (Calif.) and Rep. Carolyn McCarthy (N.Y.), who have pushed for the ban before in part because of their personal histories with gun violence.

"This is a tough battle," Feinstein said at the start of an elaborately-staged event on Capitol Hill to unveil the bill.

Feinstein and McCarthy were joined at the event by House and Senate Democrats cosponsoring the measure, representatives of gun control groups, survivors of mass shootings in Arizona, Colorado and at Virginia Tech, Philadelphia Mayor Michael Nutter and Charles H. Ramsey, Philadelphia Police Commissioner.

Ramsey, the former police chief of Washington, D.C., also discussed the details of 10 assault weapons displayed at the event, similar to those used in some of the most recent mass shootings.

"If the slaughter of 20 babies does not capture and hold your attention, then I give up, because I don't know what else will," Ramsey told the crowd. "We have to pass legislation, we can't allow the legislation to get so watered down and filled with loopholes that it is meaningless and won't do anything."

Then, turning to the weapons, Ramsey said: "Look at this and tell me why any of this needs to be on the streets of our cities. ... How are you going to go hunting with something like that? If you kill something, there’s nothing left to eat."

Feinstein later explained that the weapons displayed were in the lawful possession of unnamed law enforcement agencies as evidence.

Supporters face an uphill climb in a Congress filled with Republicans and moderate Democrats who support Second Amendment rights and rely on political support from the National Rifle Association and other gun groups to win reelection.

The NRA responded with a statement:

Senator Feinstein has been trying to ban guns from law-abiding citizens for decades. It's disappointing but not surprising that she is once again focused on curtailing the Constitution instead of prosecuting criminals or fixing our broken mental health system. The American people know gun bans do not work and we are confident Congress will reject Senator Feinstein's wrong-headed approach.

Regardless, Feinstein and McCarthy plan to press ahead.

Most Americans support tough new measures to counter gun violence, including banning assault weapons, according to the latest Washington Post-ABC News poll. In the poll, 58 percent of Americans support the ban, which expired in 2004 after 10 years; 39 percent oppose it. Some 45 percent of gun-owning households also support the ban.

McCarthy, whose husband was killed and son wounded in the 1993 Long Island Railroad shooting, has reintroduced the weapons ban every year since it expired. She said her office has received much more support for her efforts since the deadly shooting  in Newtown.

“The American people are on our side this time, and we do outnumber some of the people who are fighting against us this time,” McCarthy said Wednesday, citing new support from parents, medical professionals and labor unions that she declined to name.

"This is different this time, people are more open to it," she added. "What we keep hearing [from voters] is [go for] the assault weapons ban, so we’ll go for it.”

Feinstein, who became San Francisco mayor in 1978 after the assassinations of Mayor George Moscone and Supervisor Harvey Milk, said she has voiced her displeasure with Senate Majority Leader Harry M. Reid (D-Nev.) after he recently told a Nevada television station that, given the current political environment, it might be futile to move an assault weapons ban through Congress.

Since then, Reid has sounded more open to gun control measures. “This is an issue that we’re not going to run from,” he told reporters Tuesday. “It’s an issue we need to talk about. . . . It may not be everything everyone wants. But I hope it has some stuff in there that’s really important.”

House Republican leaders say they won't consider any gun-related legislation until the Senate takes action. This week, Sens. Patrick J. Leahy (D-Vt.) and Kirsten Gillibrand (D-N.Y.) unveiled plans to make gun trafficking and straw purchases a federal crime, and Sen. Frank Lautenberg (D-N.J.) relaunched plans to close a loophole in federal law that permits gun buyers to purchase weapons without a federal background check from private gun dealers and to ban high-capacity magazines holding more than 10 rounds of ammunition.

The proposals will be considered next Wednesday at a Senate Judiciary Committee hearing on gun control. Leahy, who chairs the panel, has said he's eager to consider a wide range of proposals before moving legislation through the Senate.

The last time Congress approved the federal ban on assault weapons was 1994, when Feinstein faced her toughest reelection race and McCarthy was a nurse — and registered Republican — grieving the death of her husband and helping her son recover from his wounds.

In addition to growing support for stricter gun laws, McCarthy noted that President Obama's campaign operation, recently renamed Organizing for Action, is planning to help mobilize supporters.

“I would love his e-mail list," McCarthy said of Obama's support network.

“Each of us can work as hard as we can, but unless [Obama is] out there selling it," the bill won't advance, McCarthy said. "Hopefully they learned their lessons from the health-care bill.”

Scott Clement contributed to this report.

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