WASHINGTON -- Sen. Dianne Feinstein, D-Calif., and a group of Capitol Hill lawmakers joined law enforcement officials, mayors, clergy and victims of gun violence Thursday to offer a new ban on assault weapons and high-capacity magazines like the ones used in recent mass shootings in Connecticut and Colorado.

But one thing was missing from the event, and necessary for the proposal to have any chance of passage: Republicans. None of the 14 Senate co-sponsors on the bill are Republicans.

Nor are any Republicans expected to attend a Friday roundtable discussion on gun violence in Richmond, Va., led by Vice President Joe Biden, who spearheaded the White House effort to develop a gun control plan.

Two Virginia Democrats, Sen. Tim Kaine and Rep. Bobby Scott, will participate in the discussion in the state capital, home to Republican House Majority Leader Eric Cantor. Biden’s office said Cantor declined an invitation to attend.

Republicans control the House of Representatives and hold 45 seats in the Senate, meaning that no gun legislation can get through Congress without some GOP support. And that assumes that all Democrats fall in line first, which also is not likely, given the highly charged politics surrounding the issue.

Despite supporters’ impassioned pleas to act, banning assault weapons and high-capacity ammunition clips face major obstacles.

“Getting this bill signed into law will be an uphill battle, and I recognize that – but it’s a battle worth having,” Feinstein said. “We must balance the desire of a few to own military-style assault weapons with the growing threat to lives across America.”

Assault weapons were used in shootings last month in Newtown, Conn., where 20 children and seven adults, including the shooter, were killed at an elementary school; and last year in Aurora, Colo., where a dozen people were killed at a movie theater.

Feinstein led the 1994 effort to get an assault weapons ban passed in a Congress then controlled by Democrats. President Bill Clinton signed the law, but it expired a decade later and was not renewed by the Republican-controlled Congress. Since 2004, 350 people have been killed and more than 450 people injured by such weapons, according to Feinstein.

Dan Gross, president of the Brady Campaign to Prevent Gun Violence, praised her bill and said in a statement that the gun-control group would work with Congress to get it passed.

“The majority of Americans support the measures, and we will continue in our role of bringing the voice of the American public to Congress,” he said.

A new survey by the Pew Research Center for the People and the Press found that 39 percent of the public viewed the new White House gun control plan, which includes an assault weapons ban, as “about right,” according to Pew. Thirty-one percent said it goes too far.

Feinstein’s legislation, which has the backing of a variety of police chiefs, mayors, teachers, and religious and medical organizations, would exempt more than 2,200 hunting and sporting rifles. But the National Rifle Association, which opposed the original assault weapons ban, said it was confident that Congress would reject the new legislation.

“Sen. Feinstein has been trying to ban guns from law-abiding citizens for decades,” it said in a statement. “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.”

Gun control supporters have not been able to match the NRA’s might. According to the Sunlight Foundation, a government spending watchdog group in Washington, the NRA hired 42 lobbyists last year, including a Democrat, former Rep. Allen Boyd of Florida. Gun control groups employed eight.

Gun control has always been a politically radioactive subject for Congress. Though he is championing the issue now, President Barack Obama barely mentioned gun control during either of his presidential campaigns.

But the killing of 20 elementary school children last month seemed to crystallize the anger and determination to do something about guns that gun control activists and many in the general public regularly express after every high-profile shooting, only to see it dissipate as time passes.

But Feinstein and the co-sponsors of her bill said they believe that Newtown was the last straw.

“If 20 dead children in Newtown wasn’t a wakeup call that these weapons of war don’t belong on our streets, I don’t know what is,” she said.

Sen. Dick Durbin, D-Ill., said he grew up in a hunting culture in southern Illinois and that the legislation, which he is co-sponsoring, would protect sportsmen.

“We need responsible hunters and sportsmen to speak up,” Durbin said.

In California, supporters include Democratic Sen. Barbara Boxer and the mayors of numerous cities, including Sacramento, Los Angeles, San Francisco, Oakland, San Jose and San Diego.

But Thomas Del Beccaro, chairman of the California Republican Party, echoing the NRA, said such laws fail to address the role mental illness has played in mass shootings, including those in Newtown, Aurora and Tucson.

“That discussion is not being held, and we need to talk about it,” he said. “We need to talk more about mental health.”

Anita Kumar in Washington contributed to this article. David Lightman contributed from Charlotte, N.C.

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