WASHINGTON — President Barack Obama is launching the nation’s most sweeping effort to curb gun violence in nearly two decades, urging a reluctant Congress to ban military-style assault weapons as well as the kinds of high-capacity ammunition magazines used in last month’s massacre of 20 elementary school children in Newtown, Conn.

The broad package Obama will announce Wednesday will also include efforts to stop bullying and boost availability of mental health services. It’s expected to include more than a dozen steps the president can take on his own through executive action. Those measures will provide a pathway for skirting opposing lawmakers, but they will be limited in scope, and in some cases, focused simply on enforcing existing laws.

But Congress would have to approve the bans on assault weapons and ammunition magazines holding more than 10 bullets, along with a requirement for universal background checks on gun buyers. Some gun control advocates worry that opposition from Republicans and conservative Democrats, as well as the National Rifle Association, will be too great to overcome.

“We’re not going to get an outright ban,” Rep. Carolyn McCarthy, D-N.Y., said of limits on assault weapons. Still, McCarthy, a leading voice in Congress in favor of gun control, said she would keep pushing for a ban and hoped Obama would as well. 

White House officials, seeking to avoid setting the president up for failure, have emphasized that no single measure — even an assault weapons ban — would solve a scourge of gun violence across the country. But without such a ban, or other sweeping Congress-approved measures, it’s unclear whether executive actions alone can make any noticeable difference.

“It is a simple fact that there are limits to what can be done within existing law,” White House spokesman Jay Carney said Tuesday. “Congress has to act on the kinds of measures we’ve already mentioned because the power to do that is reserved by Congress.”

New York Gov. Andrew Cuomo on Tuesday signed into law the toughest gun control law in the nation, and the first since the Connecticut school shootings. The law includes a tougher assault-weapons ban and provisions to try to keep guns out of the hands of mentally ill people who make threats.

Obama will announce his proposals in a midday event at the White House, flanked by children who wrote to him about gun violence following the massacre of 20 students and six adults at Sandy Hook Elementary School. Law enforcement officials, mayors from across the country and supportive congressional lawmakers are also expected to attend.

According to a lobbyist briefed Tuesday, Obama will present a three-part plan focused on gun violence, education and mental health.

The president will call for a focus on universal background checks. Right now some 40 percent of gun sales happen without background checks, including by private sellers at gun shows or over the Internet, according to the Brady Campaign to Prevent Gun Violence.

The president will call for banning assault weapons and limiting ammunition magazines to 10 rounds or fewer, and also propose a federal statute to stop “straw man” purchases of guns and crack down on trafficking rings. He’ll order federal agencies to conduct more research on gun use and crimes, something Republican congressional majorities have limited through language in budget bills, the lobbyist said.

On education, Obama will call for more anti-bullying efforts; more training for teachers, counselors and principles; and providing resources for schools for more counselors and resource officers.

And on mental health, Obama will focus on more availability of mental health services, training more school counselors and mental health professionals, and mental health first aid training for first responders, according to the lobbyist briefed on the plans. The lobbyist provided the information anonymously ahead of Obama’s announcement.

Obama has pledged urgent action to prevent future mass shootings, and his plan — coming just one month after the Newtown attacks — is swift by Washington standards.

The president’s framework is based on recommendations from Vice President Joe Biden, who led a wide-ranging task force on gun violence. The vice president’s proposals included 19 steps that could be achieved through executive action.

Obama may order the Justice Department to crack down on people who lie on background checks; only a tiny number are now prosecuted. Such a step has support from the National Rifle Association, which has consistently argued that existing laws must be enforced before new ones are considered.

He also could take steps ordering federal agencies to make more data on gun crimes available and conduct more research on the issue, something Republican congressional majorities have limited through language in budget bills. And he may order tougher penalties against gun trafficking and give schools flexibility to use grant money to improve safety.

Gun control proponent Rep. Bobby Scott, D-Va., who met with Biden on Monday, said the president is also likely to take executive action to ensure better state reporting of mental health and other records that go into the federal background check database. But he, too, acknowledged there were clear limits to what Obama can do without Congress’ say-so.

“You can’t change the law through executive order,” Scott said.

The NRA released an online video Tuesday that called Obama an “elitist hypocrite” for having armed Secret Service agents protect his daughters at school while not committing to installing armed guards in all schools.

White House officials signaled that Obama would seek to rally public support for the measures he puts forward, perhaps holding events around the country or relying on Organizing for America, his still-operational presidential campaign.

“The president’s success in using this strategy, I think, is pretty notable,” Carney said of Obama’s efforts to engage the public in previous legislative fights. “He’ll absolutely continue to engage with the American people on the policy proposals he’s putting forward.” 

Still, it’s unclear how much political capital Obama will exert in pressing for congressional action.

The White House and Capitol Hill will soon be consumed by three looming fiscal deadlines, each of which is expected to be contentious. And the president has also pledged to tackle comprehensive immigration reform early this year, another effort that will require Republicans’ support and one in which Obama may be more likely to get their backing.

Kentucky Sen. Mitch McConnell, the chamber’s top Republican, has warned the White House that it will be at least three months before the Senate considers gun legislation. And Senate Majority Leader Harry Reid has said it is immigration, not gun control, that is at the top of his priority list after the fiscal fights.  

House Republican leaders are expected to wait for any action by the Senate before deciding how — or whether — to proceed with any gun measure. Publicly, House GOP leaders are being careful not to rule anything out ahead of Obama’s announcement.

“I can’t respond to any particulars because I haven’t even looked at the Biden recommendations, but I can tell you we’re all very concerned about the deaths that occurred and the innocent lives lost, and if we bear that in mind, the kinds of things we can do to help make that not happen again,” House Majority Leader Eric Cantor, R-Va., said Tuesday.

Privately, House Republicans voice skepticism that the debate will even get to the point of Senate action that would require a response by the House.

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