President Obama and Vice President Biden formally announced the specifics of their gun-control proposals in a press conference Wednesday. (Nicki Demarco/The Washington Post)

The gun-control agenda that President Obama unveiled with urgency on Wednesday now faces an uncertain fate in a bitterly divided Congress, where Republican opposition hardened and centrist Democrats remained noncommittal after a month of feverish public debate.

By pursuing an expansive overhaul of the nation’s gun laws, Obama is wagering that public opinion has evolved enough after a string of mass shootings to force passage of politically contentious measures that Congress has long stymied.

Yet there was no indication on Wednesday that the mood on Capitol Hill has changed much. Within hours of Obama’s formal policy rollout at the White House, Republicans who had previously said they were open to a discussion about gun violence condemned his agenda as violating the Second Amendment’s right to bear arms.

“I’m confident there will be bipartisan opposition to his proposal,” Sen. Lindsey O. Graham (R-S.C.) said in a statement.

The Senate plans to begin taking up Obama’s proposals next week, with the House waiting to see what the Democrat-controlled Senate passes first, congressional aides said. The Senate is likely to take a piecemeal approach, eventually holding up-or-down votes on the individual elements of Obama’s plan rather than trying to muscle through a single comprehensive bill, aides said.

President Obama proposed expansive gun-control policies aimed at curbing gun violence. The Obama administration can implement about half of the proposals, but the others — arguably some of the more critical initiatives — will require congressional approval.

Obama, in an emotional White House ceremony, outlined four major legislative proposals aimed at curbing what he called “the epidemic of gun violence in this country”: universal background checks for all gun buyers, a crackdown on gun trafficking, a ban on military-style assault weapons and a ban on ammunition magazines holding more than 10 bullets.

Obama also signed paperwork initiating 23 executive actions that include steps to strengthen the existing background-check system, promote research on gun violence and provide training in “active shooter situations.” He also nominated Todd Jones, acting director of the Bureau of Alcohol, Tobacco, Firearms and Explosives, to become the agency’s permanent director.

As important as the executive actions are, Obama said, “they are in no way a substitute” for the legislative proposals he sent to Congress.

“We have to examine ourselves in our hearts and ask yourselves: What is important?” Obama said. He added, “If parents and teachers, police officers and pastors, if hunters and sportsmen, if responsible gun owners, if Americans of every background stand up and say, enough, we’ve suffered too much pain and care too much about our children to allow this to continue, then change will — change will come.”

But on Capitol Hill, where two decades of gun-control efforts have landed in the political graveyard, leaders of Obama’s own party do not necessarily share his views.

Senate Majority Leader Harry M. Reid (D-Nev.) stopped short of embracing Obama’s proposals, calling them “thoughtful recommendations” and saying that he would “consider legislation that addresses gun violence and other aspects of violence in our society early this year.”

In contrast with his role in the major policy debates during Obama’s first term, Reid is likely to step back on guns, according to Senate Democratic aides. He will leave it to Sens. Charles E. Schumer (D-N.Y.), Dianne Feinstein (D-Calif.), Patrick J. Leahy (D-Vt.) and Frank Lautenberg (D-N.J.) to shepherd the legislation, at least for now.

Reid is concerned about the potential political impact on fellow Democrats representing rural or conservative states, and he believes gun control could become a significant issue for at least 10 of the 23 Democratic Senate seats up for grabs in 2014, aides said.

The four measures Obama presented — which, taken together, rank among the most ambitious legislative projects of his presidency — appear to have varying levels of support in Congress.

The White House and Democratic lawmakers have calculated that the assault-weapons ban — a version of which passed in 1994 but expired a decade later — has the toughest odds, according to gun-control advocates in regular contact with administration officials. Also in jeopardy, they said, is the proposal to prohibit high-capacity magazines.

But a broad consensus seems more likely to build around universal background checks, which senior administration officials said is Obama’s top priority. Schumer said the idea is “at the sweet spot” of what is politically possible.

The gun trafficking proposal, which would impose new penalties on those who buy multiple firearms and hand them off to criminals, also could find majority support.

“If you are left in a position of having to oppose universal background checks and a firearms trafficking statute, that’s tough for responsible Republicans,” said Matt Bennett, a senior vice president at Third Way, a centrist think tank.

House Speaker John A. Boehner (R-Ohio) adopted a wait-and-see approach Wednesday. His spokesman, Michael Steel, said House committees will consider Obama’s proposals and “if the Senate passes a bill, we will also take a look at that.”

But the statements from many other Republicans at both ends of the Capitol were far tougher. Rep. Steve Stockman (R-Tex.), who has threatened to initiate impeachment proceedings against Obama, condemned what he described as Obama’s “anti-gun sneak attack” and promised a legislative battle to protect “the God-given right to keep and bear arms.”

A potential presidential candidate, Sen. Marco Rubio (R-Fla.), said: “President Obama is targeting the Second Amendment rights of law-abiding citizens instead of seriously addressing the real underlying causes of such violence.”

And Sen. Charles E. Grassley (R-Iowa), who last week said he would be open to some form of gun control, said on Wednesday that Obama’s executive actions amounted to a “power grab” to “poke holes in the Second Amendment.”

No Republican lawmakers attended Wednesday’s White House ceremony. The only vestige of bipartisanship came when Obama invoked former president Ronald Reagan. He noted that Reagan, “one of the staunchest defenders of the Second Amendment,” wrote to Congress in 1994 to urge support for the assault-weapons ban.

Obama acknowledged that getting his proposals through Congress “will be difficult,” making a veiled reference to powerful lobbying groups such as the National Rifle Association.

“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,” Obama predicted. “And behind the scenes, they’ll do everything they can to block any common-sense reform and make sure nothing changes whatsoever.”

In its official response, the NRA adopted a more muted tone than it has in recent weeks, saying it would work with Congress “on a bipartisan basis” to develop solutions that secure the nation’s schools and fix broken mental health systems. The statement did not specifically address Obama’s proposals, which include a $150 million school-safety initiative to help communities hire 1,000 new school resource officers.

But at a huge annual gun show in Las Vegas, the NRA said its opposition to Obama’s plans was “the fight of the century.”

“I warned you this day was coming, and now it’s here,” NRA executive vice president Wayne LaPierre wrote in a fundraising letter circulated at the trade show. “It’s not about protecting your children. It’s not about stopping crime. It’s about banning your guns . . . PERIOD!”

Gun-control advocates say their strategy will be to highlight popular support for most of Obama’s proposals and rally voters across the country to press their representatives in Congress to act.

“There’s an extraordinary disconnect between what the American public wants — including gun owners and NRA members — and what our elected officials are doing about it,” said Dan Gross, president of the Brady Campaign to Prevent Gun Violence. “It is going to be up to us, the American public, to close that disconnect.”

Obama vowed Wednesday to “put everything I’ve got into this.” In a moving event one month and two days after a gunman killed 20 small children and six adults at Sandy Hook Elementary School in Newtown, Conn., Obama was flanked by children who wrote him letters in the days after the massacre, pleading with him to do something to curb gun violence.

The president urged Americans to put pressure on their members of Congress and “get them on record” on whether they support universal background checks on gun buyers and renewal of the bans on assault weapons and high-capacity magazines.

“And if they say no, ask them why not,” Obama said. “Ask them what’s more important: Doing whatever it takes to get an ‘A’ grade from the gun lobby that funds their campaigns, or giving parents some peace of mind when they drop their child off to first grade?”

Vice President Biden, who headed the task force that developed Wednesday’s proposals, said “we have a moral obligation” to reduce the chances that tragedies such as the one in Newtown could happen again.

“I have no illusions about what we’re up against,” Biden said. But he added: “The world has changed, and it’s demanding action.”

Sari Horwitz in Las Vegas and William Branigin, Scott Wilson and Lyndsey Layton in Washington contributed to this report.