WASHINGTON — President Barack Obama is launching the nation’s most sweeping effort to curb gun violence in two decades, setting up a legislative fight with a deeply divided Congress that even some of his staunchest allies expect to fall short of its goals.

The broad package Obama will announce today, just over a month after the horrific Newtown, Conn., school shootings, is 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.

The proposals that require congressional approval will include a ban on military-style assault weapons and high-capacity ammunition magazines along with a requirement for universal background checks on gun buyers. But 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’s Assembly on Tuesday easily passed the toughest gun control law in the nation and the first since the Connecticut school shootings. The statewide measure 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. 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. Beyond the gun control measures, Biden also gave Obama suggestions for improving mental health care and addressing violent images in video games, movies and television.

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.

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.