WASHINGTON -- Sen. Dianne Feinstein, D-Calif., and other members of Congress joined law enforcement officials, mayors, clergy members and victims of gun violence Thursday to propose a ban on assault weapons and high-capacity magazines like the ones used in mass shootings in Connecticut and Colorado.Her new measure goes further than the now-lapsed 1994 law that she authored. It would prohibit the sale, import and manufacture of more than 150 weapons -- including the make of Bushmaster rifle used in the Connecticut school shootings -- and magazines that can hold more than 10 rounds.Those who legally own assault weapons could keep them. Buyers of currently owned assault weapons would be subject to criminal background checks.But one thing was missing Thursday that would be necessary for the proposal to have any chance of passing: the GOP. None of the 14 Senate co-sponsors are Republicans.Nor are any Republicans expected to attend a round-table discussion today that will be led by Vice President Joe Biden, who heads the administration's gun control effort.Two Virginia Democrats, Sen. Tim Kaine and Rep. Bobby Scott, will participate in the discussion in Richmond, which is home to Republican House Majority Leader Eric Cantor. Biden's office said Cantor declined an invitation.Republicans control the House and hold 45 seats in the Senate, meaning no gun legislation can get through Congress without some GOP support. It's also unlikely that all Democrats would back it.A ban on assault weapons and high-capacity magazines faces big hurdles. "Getting this bill signed into law will be an uphill battle, and I recognize that -- but it's a battle worth having," Feinstein said.Assault weapons were used in shootings last month in Newtown, Conn., where 20 children and eight adults, including the shooter, were killed, and last year in Aurora, Colo., where 12 people were killed at a movie theater.Feinstein led the 1994 effort to pass an assault weapons in a Congress then controlled by Democrats. President Bill Clinton signed the law, but it expired a decade later and was not renewed by a Republican-controlled Congress.Since 2004, 350 people have been killed and more than 450 have been injured by such weapons, Feinstein said.Although Feinstein dropped the idea of requiring owners of assault weapons to register, her proposal quickly drew criticism from the National Rifle Association."Sen. Feinstein has been trying to ban guns from law-abiding citizens for decades," the NRA 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. The American people know gun bans do not work and we are confident Congress will reject Sen. Feinstein's wrongheaded approach."The bill is modeled after California's tough assault weapons ban, but it would close a loophole in the state law. It would ban assault weapons that have a device -- such as a so-called bullet button -- that can be used to swiftly reload the weapons with multiple rounds.Biden speaks outBiden said Thursday that he's more concerned about limiting the number of rounds in a gun magazine than about banning assault weapons that account for a small percentage of gun deaths.Biden argued that the shooter at Sandy Hook Elementary School in Newtown could have been slowed down if he had fewer rounds in each magazine and had to reload more often. "Maybe if it took longer, maybe one more kid would be alive," Biden said during an online video chat on Google Plus.Biden said he wants a ban on assault weapons despite acknowledging that they don't account for a large percentage of gun deaths.He said a ban would solve part of the gun violence problem, particularly for police who can be outgunned by criminals with assault weapons."It is not an answer to all the problems," Biden said. But he views a ban as "a rational limitation."Meanwhile, the White House has decided to circumvent Capitol Hill as it concentrates its gun control efforts on speeches and other public appearances by President Barack Obama and Biden outside of Washington, according to officials with knowledge of the plans.With Obama's gun agenda dependent on centrist Democratic senators nervous about their re-election prospects, the administration has calculated that the president is better off helping build popular support within their states rather than negotiating directly with the lawmakers, officials said.This report includes material from The Associated Press, The Washington Post and the Los Angeles Times.