President Obama urged lawmakers to pass his gun-control agenda in an emotional and forceful speech at the White House on Thursday. (The Washington Post)

National attention returned to the Newtown, Conn., massacre Thursday as authorities unsealed warrants detailing what detectives found in searching Adam Lanza’s home. The list of items included weapons and ammunition.

Among the other items authorities said they found following the December shooting in Newtown, Conn.: a news account of a mass shooting at Northern Illinois University, knives, a bayonet, books on autism savants and Asperger’s syndrome, computers and video game consoles, and Lanza’s own report card from Sandy Hook.

Elsewhere, advocates of gun control held rallies around the country to demand that legislators support stricter background checks:

With negotiations on Capitol Hill over legislation largely stalled in recent weeks, gun-control­ groups hope that staging a “National Day to Demand Action” will help them regain momentum more than three months after the massacre at an elementary school in Newtown, Conn., sparked a national debate over gun laws.

President Obama also gave an emotional speech at the White House on Thursday afternoon, warning the country not to forget the deaths in Newtown and calling for the public to get involved:

Senate Democrats are preparing to consider several proposals that aim to limit gun violence, but a controversial ban on hundreds of weapons and parts is unlikely.

“We need everybody to remember how we felt 100 days ago and make sure that what we said at that time wasn’t just a bunch of platitudes, that we meant it,” Obama said.

(Watch the speech and read the transcript here .)

Meanwhile, New York Mayor Michael Bloomberg has begun a large advertising campaign, The Fix notes:

Bloomberg is spending $12 million — via his Mayors Against Illegal Guns group — on ads targeting 13 senators on broadening background checks for guns, a move designed to pressure them during a two-week April recess that precedes the debate on proposed new gun control measures.

Still, popular support for reform has fallen in polls since the shooting, and it isn’t clear whether the bill on background checks, scheduled to be taken up in the Senate next month, will succeed. Chris Cillizza writes at The Fix:

Major questions remain about the viability of the Senate gun proposal due in no small part to concerns from Democrats up for reelection in 2014 in conservative states — Arkansas, North Carolina, Montana, among others — about the political reverberations of voting for it.

Republicans are also offering opposition. This afternoon, Sen. Marco Rubio (R-Fla.) said he would join three other Republican senators in a filibuster of the bill, a threat announced Tuesday in a letter to Senate Majority Leader Harry M. Reid (D-Nev.) :

The inclusion of Rubio — a closely watched, well-liked conservative, who like Paul is considering a 2016 presidential run — should only add more exposure to Republican opposition to the legislation.

But unless the four Republican senators follow through on the threat, the letter is merely that — a letter. And despite potential GOP opposition, Reid and Senate Democrats may still yet be able to muster enough votes to override a threatened filibuster.

Post opinion writer Ruth Marcus discusses the proposal in detail. She argues that even slightly stricter rules on gun purchases could protect many innocent people from harm:

The real holdup in the negotiation over expanded background checks involves recordkeeping — specifically, whether private sellers should be subjected to the same requirement as licensed firearms dealers to maintain sales records.

Of course they should. In a perfect world — that is, in a world without the NRA — guns would be treated like automobiles. The government knows who owns a particular car and when, and to whom, it is transferred. These records are computerized and searchable . . .

Yet restaurants card would-be drinkers, grocery stores ask for proof of age from prospective tobacco purchasers — all without keeping records of such checks. The incentive to obey comes from the fact of the law’s existence and the risk of random sting operations.