WASHINGTON -- New York Rep. Carolyn McCarthy, whose husband was gunned down in a 1993 mass shooting on the Long Island Rail Road, has sought to outlaw so-called high-capacity ammunition magazines ever since Congress let the ban on assault-style weapons expire in 2004.

The measure never gained much traction, however, in the face of stiff opposition from the National Rifle Association and other gun rights groups. But after last week's massacre in Newtown, Conn., nearly two dozen Democrats this week quickly added their names to the bill, including several who previously have backed legislation favored by the NRA.

The wave of public horror over the slaughter of 20 young children and six adults at Sandy Hook Elementary School has spurred gun-control advocates such as McCarthy to ramp up for one of their biggest legislative battles in nearly two decades over guns and their role in American life. The fight could pose one of the most serious challenges to the National Rifle Association's political clout in years.

NRA officials did not respond to interview requests, but in a statement Tuesday on the slayings said that it is "prepared to offer meaningful contributions to help make sure this never happens again." It plans a news conference Friday in Washington.

Gun rights supporters say those pushing for restrictions should not underestimate the NRA's ability to mobilize its membership when threatened with restrictions.

"The NRA has 4.3 million dues-paying members who communicate with their representatives," said David Kopel, an associate policy analyst at the Cato Institute and an NRA member. He said the guns and ammunition Democrats are pushing to outlaw are commonly used by millions of law-abiding Americans. "When the time comes for a vote, there will be more and more senators and representatives who find themselves reluctant to label their constituents evil people," he said.

However, gun-control advocates are questioning the NRA's power to sway election results - despite its reputation as a political powerhouse on Capitol Hill.

"I think the NRA is to a large extent the Wizard of Oz," said Rep. John Yarmuth, D-Ky., who this week called for comprehensive gun-control legislation. "You can't point to a lot of races where they have had an impact."

A review of campaign records shows the gun-rights group had few victories at the ballot box in November and saw its spending dramatically eclipsed by the other groups.

The NRA pumped two-thirds of its independent spending in last month's elections into defeating Obama. It spent more than $100,000 in nine Senate contests, and the NRA's favored candidates won in three. In the Ohio Senate race, it spent nearly $1 million to oppose Democratic Sen. Sherrod Brown and back Republican rival Josh Mandel. Its ads highlighted its "F" grade for Brown, citing his opposition to a slew of pro-gun bills -- including a 2005 measure that would have blocked lawsuits against gun manufacturers.

At the same time, the rise of new super PACs diluted the NRA's influence in presidential and congressional elections. The group ranked third in independent spending during the 2008 campaign, before the Supreme Court's 2010 Citizens United decision paved the way for unlimited corporate and union money for independent advertising. This year, despite spending more money, the NRA's rank fell to No. 12, according to data compiled by the non-partisan Center for Responsive Politics.

By comparison, two GOP-aligned groups created by former White House strategist Karl Rove reported more than $175 million in last-minute independent spending, roughly 10 times the amount reported by the NRA.

"When you have $2 billion spent in a presidential election, the $12 million to $20 million that the NRA can put up is chump change," said Mark Glaze, director of Mayors Against Illegal Guns, a group co-founded by New York Mayor Michael Bloomberg that promotes tougher gun regulation.

In a post-election assessment, the NRA's lobbying arm touted gains in state contests and the passage of pro-gun amendments to state constitutions in Louisiana, Idaho, Kentucky and Nebraska, but acknowledged that little had changed in the nation's capitol.

"Prior to Tuesday, our country had an anti-gun president, a questionable U.S. Senate majority, and a pro-gun U.S. House majority," the group said in a statement. "Today, America still has an anti-gun president, questionable U.S. Senate majority, and pro-gun U.S. House majority. As Ronald Reagan once famously said: 'Status quo is, you know, Latin for the mess we're in.' "

Records show the NRA spends far more on mobilizing its members than it does on political advertising.

Its 2010 tax return shows the not-for-profit group spent $57 million that year on "member communications" and another $50 million on staff salaries and benefits - swamping the $9.7 million it told the Federal Election Commission it had invested in political contributions and independent spending during that year's midterm elections for Congress.

"Each member of Congress represents 600,000 people, and if they get 100 calls, they pay attention," said David Yassky, a New York taxi and limousine commissioner, who helped draft the controversial 1994 assault weapons ban as an aide to then-congressman Charles Schumer, D-N.Y. "That's why the NRA has been so powerful."

In the 1994 elections, more than two dozen incumbent Democrats who had voted for the ban lost their seats, including House Judiciary Committee Chairman Jack Brooks, D-Texas.

The gun debate has not been central to federal elections for more than a decade, said Richard Feldman, a former NRA lobbyist who now serves as president of the Independent Firearms Owners Association. "I'm not going out on much of a limb to say that in 23 months, it will make a huge difference in many races in this country in a way it hasn't for years."

In all, 22 new supporters had joined McCarthy's bill this week. None was Republican.

Rep. Rick Larsen, D-Wash., a self-described Second Amendment supporter, added his name to the bill Tuesday. Last year, he was one of 43 Democrats who backed an NRA-supported bill to make a state permit to carry a concealed weapon valid in almost every other state.

McCarthy's measure would limit to 10 the number of rounds in a single magazine, requiring the shooter to pause to reload.

Investigators in the Newtown slayings said they recovered "numerous" empty 30-round magazines for the Bushmaster rifle used in that shooting.

Larsen said his views on gun restrictions had "evolved" this year, starting with a rampage in an Aurora, Colo., theater in July that killed 12 and injured 57.

"Aurora really opened my eyes to the issues, and Newtown opened my heart to it," he said.

USA Today