WASHINGTON — Braced for a fight, President Barack Obama on Wednesday unveiled the most sweeping proposals for curbing gun violence in two decades, pressing a reluctant Congress to pass universal background checks and bans on military-style assault weapons and high-capacity ammunition magazines like the ones used in the Newtown, Conn., school shooting.

A month after that horrific massacre, Obama also used his presidential powers to enact 23 measures that don’t require the backing of lawmakers. The president’s executive actions include ordering federal agencies to make more data available for background checks, appointing a director of the Bureau of Alcohol, Tobacco, Firearms and Explosives, and directing the Centers for Disease Control to research gun violence.

But the president, speaking at White House ceremony, focused his attention on the divided Congress, saying only lawmakers could enact the most effective measures for preventing more mass shootings.  

“To make a real and lasting difference, Congress must act,” Obama said. “And Congress must act soon.”

The president vowed to use “whatever weight this office holds” to press lawmakers into action on his $500 million plan. He is also calling for improvements in school safety, including putting 1,000 police officers in schools and bolstering mental health care by training more health professionals to deal with young people who may be at risk.

The National Rifle Association promptly took issue with Obama’s proposals, and even supportive lawmakers said the president’s gun control measures face long odds in Congress.

“Attacking firearms and ignoring children is not a solution to the crisis we face as a nation,” the NRA said in a statement. “Only honest, law-abiding gun owners will be affected and our children will remain vulnerable to the inevitability of more tragedy.”

House Speaker John Boehner’s office was non-committal to the president’s package, but signaled no urgency to act on the legislative proposals. “House committees of jurisdiction will review these recommendations,” Boehner spokesman Michael Steel said. “And if the Senate passes a bill, we will also take a look at that.”

Senate Judiciary Committee Chairman Patrick Leahy said ahead of Obama’s presentation that he didn’t know whether an assault weapons ban could pass the Senate, but said there are some measures that can, such as improved background checks.

“There are some who say nothing will pass. I disagree with that,” Leahy, D-Vt., told students at Georgetown University Law Center. “What I’m interested in is what we can get.”

Senate Majority Leader Harry Reid, D-Nev., called Obama’s package “thoughtful recommendations” and said the Senate would consider legislation addressing gun violence early this year.

“The tragedy at Sandy Hook was just the latest sad reminder that we are not doing enough to protect our citizens - especially our children - from gun violence and a culture of violence, and all options should be on the table moving forward,” he said.

Republican National Committee Chairman Reince Priebus dismissed Obama’s measures as “an executive power grab.”

“He paid lip service to our fundamental constitutional rights,” Priebus said of the president, “but took actions that disregard the Second Amendment and the legislative process.”

Acknowledging the tough fight ahead, Obama said there will be pundits, politicians and special interest groups that will seek to “gin up fear” that the White House wants to take away the right to own a gun.

“Behind the scenes, they’ll do everything they can to block any commonsense reform and make sure nothing changes whatsoever,” he said. “The only way we will be able to change is if their audience, their constituents, their membership says this time must be different, that this time we must do something to protect our communities and our kids.”

The president was flanked by children who wrote him letters about gun violence in the weeks following the Newtown shooting. Families of those killed in the massacre, as well as survivors of the shooting, were also in the audience, along with law enforcement officers and congressional lawmakers.

“This is our first task as a society, keeping our children safe,” Obama said. “This is how we will be judged.”

Seeking to expand the impetus for addressing gun violence beyond the Newtown shooting, the president said more than 900 Americans have been killed by guns in the month since the elementary school massacre.  

“Every day we wait, the number will keep growing,” he said.

The White House has signaled that Obama could launch a campaign to boost public support for his proposals. Nearly six in 10 Americans want stricter gun laws in the aftermath of the Newtown shooting, with majorities favoring a nationwide ban on military-style, rapid-fire weapons and limits on gun violence depicted in video games, movies and TV shows, according to a new Associated Press-GfK poll.

A lopsided 84 percent of adults would like to see the establishment of a federal standard for background checks for people buying guns at gun shows, the poll showed.

The president based his proposals on recommendations from an administration-wide task force led by Vice President Joe Biden. His plan marks the most comprehensive effort to address gun violence since Congress passed the 1994 ban on high-grade, military-style assault weapons. The ban expired in 2004, and Obama wants lawmakers to renew and expand it.

Other measures Obama wants Congress to take up include limiting high-capacity ammunition magazines and requiring background checks for all gun buyers in an attempt to close the so-called “gun-show loophole” that allows people to buy guns at trade shows and over the Internet without submitting to background checks.

Obama also intends to seek confirmation for B. Todd Jones, who has served as acting director of the Bureau of Alcohol, Tobacco, Firearms and Explosives since 2011.

The president’s plan does little to address violent images in video games, movies and entertainment, beyond asking the CDC to study their impact on gun crimes. Some pro-gun lawmakers who are open to addressing stricter arms legislation have insisted they would do so only in tandem with recommendations for addressing violence in entertainment.  

The president’s long list of executive orders also include:

— Ordering tougher penalties for people who lie on background checks and requiring federal agencies to make relevant data available to the federal background check system.

— Ending limits that make it more difficult for the government to research gun violence, such as gathering data on guns that fall into criminal hands.

— Requiring federal law enforcement to trace guns recovered in criminal investigations.

— Giving schools flexibility to use federal grant money to improve school safety, such as by hiring school resource officers.

— Giving communities grants to institute programs to keep guns away from people who shouldn’t have them.