President Barack Obama launched the most ambitious gun-control effort in nearly two decades on Wednesday, calling on a divided Congress to pass a raft of policy proposals little more than a month after a gunman fatally shot 20 first-graders and six educators at an elementary school in Connecticut.

Obama announced the package of legislative proposals and 23 executive actions five days before he is sworn in for his second term, ensuring that gun control, a subject that made virtually no appearance on the presidential campaign trail or in his first term, will play a prominent role alongside fiscal and immigration debates in the 113th Congress. But with House Republicans showing little willingness to embrace new restrictions on firearms, the president also kicked off a national lobbying campaign designed to increase public pressure on lawmakers to enact his policy goals.

“This is our first task as a society: keeping our children safe. This is how we will be judged,” Obama said during an appearance with Vice President Joseph R. Biden Jr. and advocates, including four children who wrote to him to urge action in the aftermath of the Connecticut shooting.

“The most important changes we can make depend on congressional action,” he added. “They need to bring these proposals up for a vote, and the American people need to make sure that they do.”

Key House Republicans, however, issued cool responses to the president’s call for action.

“House committees of jurisdiction will review these recommendations,” said Michael Steel, a spokesman for House Speaker John A. Boehner, R-Ohio. “And if the Senate passes a bill, we will also take a look at that.”

House Judiciary Chairman Robert W. Goodlatte, R-Va., said Republicans “welcome the recommendations,” but he also struck a cautionary note for Obama.

“Good intentions do not necessarily make good laws, so as we investigate the causes and search for solutions, we must ensure that any proposed solutions will actually be meaningful in preventing the taking of innocent life and that they do not trample on the rights of law-abiding citizens to exercise their constitutionally guaranteed rights,” Goodlatte said.

Guns, Ammunition, Background Checks

The president called on Congress to renew and strengthen the federal assault weapons ban that expired in 2004, including a prohibition on the future manufacture of all ammunition magazines containing more than 10 rounds.

He is expected to throw his support behind a legislative proposal set to be introduced next week by Sen. Dianne Feinstein, D-Calif., who has said her bill will ban the future manufacture of at least 100 different kinds of semiautomatic guns “by name” and also will place new restrictions on the millions of such firearms currently in private possession by subjecting them to registration and other requirements. A senior administration official emphasized that there are no plans to confiscate any such firearms already in circulation.

“We’re not going to go after the existing stock of weapons and magazines,” the official said. “We’re going to limit the manufacture of assault weapons and clips going forward.”

Obama also urged lawmakers to require that all gun sales be subject to background checks in order to ensure that prospective buyers are not within the existing categories of disqualified purchasers, such as those with criminal histories or those facing mental health problems. Background checks now capture about 60 percent of all gun sales but are not conducted for private, person-to-person sales, including those at gun shows.

The background check proposal is the top policy priority of the Brady Campaign to Prevent Gun Violence, a prominent advocacy group that is working closely with the administration and is seen as the biggest counterweight to the powerful National Rifle Association.

Obama also announced that he would ask Congress to pass a federal gun trafficking law and to confirm the acting director of the Bureau of Alcohol, Tobacco, Firearms and Explosives, Todd Jones, on a permanent basis. The agency has lacked a permanent director for six years, and Jones has juggled his interim position with another job as a federal prosecutor in Minnesota.

Among the executive actions that Obama took immediately Wednesday was a lifting of a congressionally ordered ban on funding for research on guns.

The timetable for the president’s agenda also began to come into focus on Wednesday. Senate Judiciary Chairman Patrick J. Leahy, D-Vt., said his panel will start hearings on gun control in two weeks.

“We should do as much as we can as quickly as we can,” Biden said.

Biden As Emissary

Obama’s recommendations are the product of four weeks of evaluation by a Cabinet-level task force on gun violence headed by Biden, who is expected to serve as one of the administration’s key negotiators with Congress over the proposals. Biden’s task force held 22 official meetings and met with more than 220 stakeholders, a senior administration official said.

The longtime former senator from Delaware — who played a central role in the last sweeping gun-control effort to become law, in 1994 — reached a difficult compromise with Senate Minority Leader Mitch McConnell, R-Ky., in December to avert a set of steep tax hikes and spending cuts that were set to take effect this year.

But Biden will face a much tougher road to find middle ground on gun control, a subject that Democrats have largely avoided since 1994, when passage of a broad crime bill, including the original assault weapons ban, played a hand in their crushing electoral losses that fall.

Even leaving aside the outspoken opposition that many Republicans have expressed to most forms of gun control, Democrats are far from unified within their own conference when it comes to new restrictions on firearms and ammunition. Exactly half of the Democrat-led Senate has “A-minus” or higher ratings from the NRA and would not easily vote for legislative proposals that the organization is expected to strongly oppose.

Key Senate Democrats from gun-friendly states face re-election efforts in 2014 and could be targeted by their opponents for supporting the president’s agenda. The list includes Sens. Mark Begich of Alaska, Mark Pryor of Arkansas, Mark Udall of Colorado, Mary L. Landrieu of Louisiana, Max Baucus of Montana, Kay Hagan of North Carolina and Mark Warner of Virginia.

The Senate, furthermore, is seen as the far more receptive of the two chambers to the president’s proposals. Obama’s former chief of staff, Chicago Mayor Rahm Emanuel, emphasized that point during a forum at the Center for American Progress this week, when he counseled the administration on its legislative strategy in the months ahead.

“Whatever you do, start in the Senate,” Emanuel said.

Partisan Fight Ahead

Even before the president spoke on Wednesday, it was clear that some of Obama’s legislative proposals — and even some of his executive actions — will face strong headwinds.

Iowa Sen. Charles E. Grassley, the top Republican on the Senate Judiciary Committee, said in a home-state television interview that he probably would oppose the reinstatement of the assault weapons ban.

Some rank-and-file Republicans said flatly that they did not trust Obama to uphold the Second Amendment, even though the president emphasized his respect for the constitutional right to “keep and bear arms” during his remarks on Wednesday.

“I don’t trust the left to back away from their lifelong commitment to destroy the Second Amendment,” Rep. Trent Franks, R-Ariz., said in an interview this week.

Rep. Steve Stockman, a Texas Republican who returned to Congress this year after previously riding into office in the so-called “Republican Revolution” of 1994, threatened to begin impeachment proceedings against Obama as a result of his executive actions, claiming that the president was unconstitutionally circumventing Congress.

The gun-rights lobby, meanwhile, sprung into action ahead of Obama’s announcement. The NRA appealed to its supporters in an email late Tuesday, asking them to “renew or upgrade” their membership ahead of what Executive Vice President Wayne LaPierre called the “fight of the century.” The group has claimed 250,000 new members since the Dec. 14 shooting in Connecticut.

Democrats are “steamrolling ahead with legislation that would ban your guns, register your ammunition purchases and even force you to register the firearms you already own with Obama’s anti-gun bureaucrats,” LaPierre wrote.

In the immediate aftermath of the Connecticut attack, the group had retreated from public view and stopped its lobbying activity on Capitol Hill, prompting some observers to speculate that the horror of the attack, in which nearly two dozen 6- and 7-year-olds lost their lives, would prompt the organization to soften its strict position against virtually all gun-control measures. Tuesday’s e-mail underscored that the group has no such intentions.

Joanna Anderson contributed to this report.