WASHINGTON – Gun-control advocates believe the massacre at Sandy Hook Elementary School, where semi-automatic weapons were 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 Barack 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 six- and seven-year-olds has sparked an outpouring of public emotion; Obama is on stronger footing to champion gun-control measures now that he has been re-elected 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 automatic 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 toward 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, a political independent, that represents 750 mayors across the country.

Among the measures that 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 of Washington, 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.