President Barack Obama teared up Friday as he addressed the nation after a mass shooting in Newtown, Conn. left 28 people dead, including 18 children. (The Washington Post)

Gun-control advocates believe the massacre at Sandy Hook Elementary School, where a semiautomatic weapon was used to gun down 20 small children and six adults, will do what so many other recent mass killings have failed to do: force President Obama and Congress to take action.

Obama, who has frustrated gun-control advocates with his timid approach to the issue, came under increased pressure this weekend from members of his own party to call for stricter limits on gun buyers.

“Pressure is mounting,” Rep. Keith Ellison (D-Minn.) said in an interview. “We have an epidemic problem. This is ridiculous. We’ve got to act as a nation . . . . I’m just going to encourage the president to get out there and insist that there needs to be some legislation passed.”

Friday’s rampage in Newtown, Conn., could be a tipping point in a national debate over gun rights that has faded in recent years. Advocates pointed to three reasons why this shooting may change the climate in Washington in a way that the one at a Colorado movie theater and the attempted assassination of Rep. Gabrielle Giffords (D-Ariz.) did not.

The almost unimaginable killing of so many 6- and 7-year-olds has sparked an outpouring of public emotion; Obama is on stronger footing to champion gun-control measures now that he has been reelected and will never again face voters, and the National Rifle Association has been weakened after spending millions of dollars backing candidates who lost.

The scale and frequency of mass shootings has grown so extreme that gun-control advocates believe they now have a strong case to make to enact strict restrictions on semiautomatic weapons. Their challenge, though, will be turning the latest wrenching moment into a winning legislative strategy.

That, they say, requires presidential leadership.

Obama, in his statement Friday responding to the Connecticut shooting, sounded angry and resolved as he recounted a string of recent incidents of gun violence.

“We’re going to have to come together and take meaningful action to prevent more tragedies like this, regardless of the politics,” he said. The president made almost the exact same pledge Saturday during his weekly radio address.

But Obama has not provided any specific proposals, and White House officials would not say what the president meant by “meaningful action.”

The gun issue would compete for Obama’s attention with his top priorities, such as the ongoing fiscal fight with Congress and his plans to push comprehensive immigration reform early next year.

In the past, Obama has said he supports reinstating an assault weapons ban and measures to toughen background checks on gun buyers. But his administration has not proposed any new legislation to accomplish those goals.

The president’s inaction has frustrated gun-control advocates, who say they were newly emboldened following Friday’s shooting.

“If having dozens of people gunned down in an elementary school doesn’t motivate Washington to do even the easy things they can do, it’s not clear what will,” said Mark Glaze, director of Mayors Against Illegal Guns, a group chaired by New York Mayor Michael Bloomberg (I) that represents 750 mayors across the country.

Kristen Rand, legislative director for the Violence Policy Center, a gun-control advocacy group, said she senses a change in the political atmosphere. “The general public is reaching the point of being fed up, and I think this will push it over the edge,” she said.

On Saturday, with the horror in Newtown still fresh, elected officials sounded impassioned calls for immediate action.

“Congress has held numerous hearings, all kinds of speeches, all kinds of accusations in the Benghazi matter. Let’s see what the Congress does now, and certainly the president will provide the kind of leadership that is necessary,” Philadelphia Mayor Michael Nutter (D) said on MSNBC. “We don’t need any more speeches. We need action.”

Rep. John B. Larson (D-Conn.) said in a statement Saturday that “to do nothing in the face of continuous assaults on our children is to be complicit in those assaults.”

“There may not be a single cure-all for the violence in our nation, however we must start the process and begin the deeper and longer conversations that need to take place. Politics be damned,” he said.

Among the measures that Larson and other Democratic lawmakers have suggested are requiring background checks for all gun sales, closing the terrorist watch-list loopholes, banning
high-capacity ammunition magazines and reinstating the assault weapons ban that expired in 2004.

It is unclear, however, whether any of those laws might have made a difference in the Connecticut shooting.

Republicans who oppose toughening gun laws, as well as those Democrats who enjoy the backing of the NRA, have been largely silent on the issue since Friday’s shooting. NRA officials have not commented publicly.

Rep. Cathy McMorris Rodgers (Wash.), the fourth-ranking House Republican, said Congress should be “careful” about suggesting new gun laws.

“We need to look at what drives a crazy person to do these kind of actions and make sure that we’re enforcing the laws that are currently on the books. And yes, definitely, we need to do everything possible to make sure that something like this never happens again,” she said in a C-SPAN “Newsmakers” interview taped Friday.

Any legislative effort could face stiff resistance not only among Republicans but also among some Democrats. Senate Majority Leader Harry M. Reid (D-Nev.) has promoted his NRA support, as do many of the senators whose elections in 2006 put Democrats in the majority.

In July, after the movie theater shooting in Aurora, Colo., Democratic congressional leaders had a hushed response that illustrated how fearful they had become of the NRA’s political clout. At the time, Sen. Charles E. Schumer (D-N.Y.), who once sponsored the now-lapsed assault weapons ban, acknowledged that there was no political path for gun legislation.

But now, after the NRA spent more than $11 million attacking Obama and other Democrats only to see most win reelection, there are signals that their fear has ebbed.

“Perhaps an awful tragedy like this will bring us together so we can do what it takes to prevent this horror from being repeated again,” Schumer said in a statement Friday.

Ellison, appearing on MSNBC, said Saturday: “We’re not afraid of the NRA. We’re ready to confront them.”