WASHINGTON — Conceding “this will be difficult,” President Barack Obama urged a reluctant Congress on Wednesday to require background checks for all gun sales and ban both military-style assault weapons and high-capacity ammunition magazines in an emotion-laden plea to curb gun violence in America.

The president’s sweeping, $500 million plan, coming one month after the school massacre in Connecticut, marks the most comprehensive effort to tighten gun laws in nearly two decades. But his proposals, most of which are opposed by the National Rifle Association, face a doubtful future in a divided Congress where Republicans control the House.

Seeking to circumvent at least some opposition, Obama signed 23 executive actions Wednesday, including orders to make more federal data available for background checks and end a freeze on government research on gun violence. But he acknowledged that the steps he took on his own would have less impact than the broad measures requiring approval from Capitol Hill.

“To make a real and lasting difference, Congress, too, must act,” Obama said, speaking at a White House ceremony with school children and their parents. “And Congress must act soon.”

The president’s announcements capped a swift and wide-ranging effort, led by Vice President Joe Biden, to respond to the deaths of 20 children and six adults at Sandy Hook Elementary School in Newtown, Conn. But Obama’s gun control proposals set him up for a tough political fight with Congress as he starts his second term, when he’ll need Republican support to meet three looming fiscal deadlines and pass comprehensive immigration reform.

“I will put everything I’ve got into this, and so will Joe,” the president said. “But I tell you, the only way we can change is if the American people demand it.”

Key congressional leaders were tepid in their response to the White House proposals.

Republican House Speaker John Boehner’s office signaled no urgency to act, with spokesman Michael Steel saying only that “House committees of jurisdiction will review these recommendations. And if the Senate passes a bill, we will also take a look at that.”

Senate Majority Leader Harry Reid, D-Nev., said he was committed to ensuring that the Senate will consider gun violence legislation “early this year.” But he did not endorse any of Obama’s specific proposals.

The president vowed to use “whatever weight this office holds” to fight for his recommendations. He’s likely to travel around the country in the coming weeks to rally public support and could engage his still-active presidential campaign operation in the effort. But he’ll have to overcome a well-financed counter-effort by the NRA.

“This will be difficult,” Obama acknowledged. “There will be pundits and politicians and special interest lobbyists publicly warning of a tyrannical, all-out assault on liberty — not because that’s true, but because they want to gin up fear or higher ratings or revenue for themselves.”

The president, speaking in front of an audience that included families of some of those killed in Newtown, said 900 Americans had lost their lives to gun violence in the four weeks since the school shootings.

“We can’t put this off any longer,” Obama declared. “Every day we wait, the number will keep growing.”

Many Democrats say an assault weapons ban faces the toughest road in Congress. Obama wants lawmakers to reinstate the expired 1994 ban on the high-grade weapons, and strengthen the measure to prevent manufacturers from circumventing the prohibition by making cosmetic changes to banned guns.

The president is also likely to face opposition to his call for Congress to limit ammunition magazines to 10 rounds.

But Democrats are hopeful they can build consensus around the president’s call for universal background checks. The Brady Campaign to Prevent Gun Violence says 40 percent of gun sales are conducted with no criminal background checks, such as in some instances at gun shows or by private sellers over the Internet or through classified ads.

The NRA is opposed to all three measures. In a statement Wednesday, the gun lobby said, “Only honest, law-abiding gun owners will be affected” by Obama’s efforts and the nation’s children “will remain vulnerable to the inevitability of more tragedy.”

And on the eve of Obama’s announcement, the NRA released an online video accusing him of being an “elitist hypocrite” for sending his daughters to school with armed Secret Service agents while opposing having guards with guns at all U.S. schools.

White House spokesman Jay Carney called the video “repugnant and cowardly.”

The president’s proposals did include a $150 million request to Congress that would allow schools to hire 1,000 new police officers, counselors and psychologists. The White House plan also includes legislative and executive action to increase mental health services, including boosting funding for training aimed at getting young people into treatment more quickly.

A lopsided 84 percent of Americans back broader background checks, according to a new Associated Press-GfK poll. Nearly six in 10 Americans want stricter gun laws, the same poll showed, with majorities favoring a nationwide ban on military-style weapons and limits on gun violence depicted in video games, movies and TV shows.

The NRA and pro-gun lawmakers have long suggested that violent images in video games and entertainment are more to blame for mass shootings than the availability of guns. But Obama’s proposals do little to address that concern, other than calling on the Centers for Disease Control and Prevention to research links between violent images and gun attacks.

Government scientists have been prohibited from researching the causes and prevention of gun violence since 1996, when a budget amendment was passed that barred researchers from spending taxpayer money on such studies.

The administration is calling on Congress to provide $10 million for expanded research.

Obama also wants lawmakers to ban armor-piercing ammunition, except for use by the military and law enforcement. And he’s asking them to create stiffer penalties for gun trafficking, to provide $14 million to help train police officers and others to respond to shootings, and to approve his nominee to run the Bureau of Alcohol, Tobacco, Firearms and Explosives.

One of the president’s executive actions on Wednesday was to nominate B. Todd Jones to head the ATF, which has been without a permanent director since 2006. Jones has served as the bureau’s acting director since 2011.