President Obama on Wednesday urged Congress to vote on measures banning the sale of assault weapons and high-capacity ammunition magazines and requiring background checks before any firearm sale, part of an emerging White House response to a massacre last week at a Connecticut elementary school.

The potential gun-control measures, which will be the focus of a working group led by Vice President Biden, mark the most specific proposals to date from Obama to deal with what he called a gun violence epidemic plaguing the United States.

The move pleased gun-control advocates and many Democrats who have been disappointed with Obama’s inaction on the issue during his first term, which has included four mass shootings of civilians.

Obama told reporters at the White House that he expected the group, which will feature key Cabinet secretaries, to bring him recommendations in January. He said he intends to discuss the issue in his State of the Union address.

With the anger and sadness over Friday’s massacre at a Newtown, Conn., school driving the issue at a busy moment in Washington, Obama vowed to act on concrete proposals “without delay” as several Hill Democrats urged Congress to do the same.

“This time, the words need to lead to action,” Obama said. He added: “We know this is a complex issue that stirs deeply held passions and political divides. . . . But the fact that this problem is complex can no longer be an excuse for doing nothing.”

The task force comes after a gunman with semiautomatic weapons killed 20 children and six employees at Sandy Hook Elementary School. Adam Lanza, 20, fatally shot his mother at their home before the attack at the school; he then killed himself.

White House officials say the package of proposals is almost certain to include new restrictions on guns, particularly assault weapons and high-capacity magazines. But aides said it will also probably include mental-health initiatives and, as Obama noted Wednesday, a close look “at a culture that, all too often, glorifies guns and violence.”

Although background checks are required for many weapons purchases, not all buyers are subject to investigation, including people shopping at gun shows.

Obama reiterated his support for the Second Amendment, which guarantees the right to bear arms but said the country’s leaders need to find ways to keep “weapons of war” out of the hands of the irresponsible few.

“There is a big chunk of space between what the Second Amendment means and having no rules at all, and that space is what Joe’s going to be working on to try to identify where we’ll find some common ground,” Obama said.

When a reporter asked “Where have you been?” on the gun issue since taking office, Obama grew defensive, noting that he has dealt with a financial collapse, the struggling auto industry, health care, two wars and other major issues.

I don’t think I’ve been on vacation,” Obama said. “I think all of us have to do some reflection on how we prioritize what we do here in Washington.”

New York Mayor Michael R. Bloomberg (I), a leading advocate for gun-control legislation, said Wednesday that he was “very encouraged” by Obama’s remarks. “The country needs his leadership if we are going to reduce the daily bloodshed from gun violence that we have seen for too long,” Bloomberg said in a statement.

Dan Gross, president of the Brady Campaign to Prevent Gun Violence — which gave Obama an “F” rating earlier in his term — said he was “very pleased” with the president’s remarks.

Obama said he tapped Biden to lead the administration’s efforts in part because of his role during more than three decades in the Senate as a leading advocate of gun legislation.

Biden helped write the 1994 crime bill, which included an assault-weapons ban that expired in 2004. Sen. Dianne Feinstein (D-Calif.) and other lawmakers have said they will introduce legislation reinstating the measure.

On Monday, Obama directed Biden and several Cabinet members — including Attorney General Eric H. Holder Jr., Education Secretary Arne Duncan and Health and Human Services Secretary Kathleen Sebelius — to formulate a multifaceted set of proposals. Those Cabinet members and Homeland Security Secretary Janet Napolitano will participate in Biden’s task force, which kicks off Thursday with a meeting of law enforcement officials.

In Connecticut on Wednesday, Sen. Richard Blumenthal (D), the state’s former attorney general, said in an interview that there is evidence pointing to a motive in the slayings but that the investigation is in progress. He declined to discuss details.

“There is a complexity and difficulty to the investigation resulting from the unique circumstances,” said Blumenthal, who is in Connecticut attending funerals. “As to making public facts, there may be sensitivity to the grieving of families and the need to be respectful.”

On Capitol Hill, Sen. Scott Brown (Mass.) became the first Republican senator to express support for a federal ban on assault weapons. “What happened in Newtown, where those children were subject to that level of violence, is beyond my comprehension,” he told a Massachusetts newspaper.

In the House, Democrats have introduced a similar assault-weapons ban, and on Wednesday called on House Speaker John A. Boehner (R-Ohio) to hold a vote on a bill banning “the transfer or possession” of high-capacity gun clips.

In an effort to demonstrate the shift in political thinking since the Newtown shooting, Democrats have tapped Rep. Mike Thompson (Calif.), a lifelong hunter and gun rights activist, to lead their gun-related efforts. Thompson said Wednesday that several Democratic proposals “certainly make sense,” including the ban on high-capacity magazines.

“I’ve been a hunter all my life, and there’s no reason to have a magazine that holds 30 shells,” Thompson said.

At an emotional news conference, Rep. Jim Himes (D-Conn.), whose district is just south of Newtown, said Republicans calling for more weapons in schools as a solution are “testosterone-laden individuals who have blood on their hands for making those comments.”

Peter Finn and Ed O’Keefe contributed to this report.