WASHINGTON — Declaring the time for action overdue, President Barack Obama promised on Wednesday to send Congress broad proposals in January for tightening gun laws and curbing violence after last week’s schoolhouse massacre in Connecticut.

Even before those proposals are drafted, Obama pressed lawmakers to reinstate a ban on military-style assault weapons, close loopholes that allow gun buyers to skirt background checks and restrict high-capacity ammunition clips.

“The fact that this problem is complex can no longer be an excuse for doing nothing,” Obama said in his most detailed comments on guns since Friday’s killing of 20 schoolchildren and six adults in Newtown, Conn. “The fact that we can’t prevent every act of violence doesn’t mean we can’t steadily reduce the violence.”

Gun control measures have faced fierce resistance in Congress for years but that may be changing now because of last week’s violence. Since then, Obama has signaled for the first time in his presidency that he’s willing to spend political capital on the issue and some prominent gun-rights advocates on Capitol Hill — Democrats and Republicans alike — have expressed willingness to consider new measures.

Still, given the long history of opposition to tighter gun laws, there is no certainty the legislation Obama backed Wednesday or the proposals he will send to Congress next month will become law.

Obama tasked Vice President Joe Biden, a longtime gun control advocate, with overseeing the administration-wide process to create those proposals. Beyond firearms’ restrictions, officials will also look for ways to increase mental health resources and consider steps to keep society from glamorizing guns and violence.

Obama’s January deadline underscores the desire among White House officials to respond swiftly to the Newtown shooting. Obama aides worry that as the shock of the shooting fades, so, too, will the prospects that pro-gun lawmakers will work with the White House to tighten restrictions.

“I would hope that our memories aren’t so short that what we saw in Newtown isn’t lingering with us, that we don’t remain passionate about it only a month later,” said Obama. He pledged to talk about gun violence in his State of the Union address.