President Obama met with families of gun violence victims at the White House on Thursday. In a speech, he said an upcoming Senate vote is the best chance in more than a decade to pass gun control legislation. (The Washington Post)

President Obama delivered a forceful and emotional plea to lawmakers Thursday to pass his gun-control agenda, saying “shame on us if we’ve forgotten” the elementary school massacre in Newtown, Conn.

Frustrated by the slow pace of progress on Capitol Hill, Obama urged passage of universal background checks and other gun-control measures while flanked by mothers of shooting victims in the East Room of the White House. He also repeatedly invoked the Dec. 14 shooting at Sandy Hook Elementary School as a cause for action.

“Less than 100 days ago that happened,” Obama said. “And the entire country was shocked. And the entire country pledged we would do something about it and this time would be different. Shame on us if we’ve forgotten. I haven’t forgotten those kids. Shame on us if we’ve forgotten.”

Obama — who spoke alongside Vice President Biden, the administration’s point person on guns — is attempting to pressure wavering lawmakers in advance of an expected Senate vote next month on his guns agenda. He urged Americans to “raise your voices and make yourselves unmistakably heard” so that lawmakers “don’t get squishy.”

“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.

But the fate of gun legislation on Capitol Hill is murky amid GOP opposition and wavering among conservative Democrats. Sen. Marco Rubio (R-Fla.), widely viewed as a 2016 presidential contender, announced Thursday that he was joining three other Senate GOP conservatives — Ted Cruz (Texas), Mike Lee (Utah) and Rand Paul (Ky.) — in threatening to filibuster Democratic gun-control legislation.

“We should look for ways to keep firearms out of the hands of criminals and the mentally ill prone to misusing them, but I oppose legislation that will be used as a vehicle to impose new Second Amendment restrictions on responsible, law-abiding gun owners,” Rubio said in a statement.

Obama’s remarks came on a “National Day to Demand Action,” when gun-control advocacy groups held more than 140 public events in 29 states designed to pressure lawmakers into voting for universal background checks.

It was also the day when newly released search warrants in the Newtown case revealed that authorities discovered more than 1,000 rounds of ammunition, an unlocked gun safe and samurai swords inside gunman Adam Lanza’s home.

The president made a veiled reference to the National Rifle Association, saying, “There are some powerful voices on the other side who are interested in running out the clock.”

“They’re doing everything they can to make all of our progress collapse under the weight of fear or frustration,” Obama said.

Obama ticked through the specific proposals under consideration in the Senate — universal background checks, a federal gun trafficking law, bans on assault weapons and high-capacity ammunition magazines — and portrayed the agenda as moderate and noncontroversial.

“All of them are consistent with the Second Amendment,” Obama said. “None of them will infringe on the rights of responsible gun owners. What they will do is keep guns out of the hands of dangerous people who put others at risk. This is our best chance in more than a decade to take common-sense steps that will save lives.”

Later, Obama added, “What we’re proposing is not radical.”

He focused particular attention on background checks, his top priority and by far the most popular of the proposals. Public polls suggest that as many as nine out of 10 Americans support the idea.

“Right now, 90 percent of Americans — 90 percent — support background checks,” Obama said. “How often do 90 percent of Americans agree on anything? It never happens.”

Obama was introduced by Katerina Rodgaard, a mother, lawyer and dance instructor from Maryland who taught a dance student killed in the 2007 shooting at Virginia Tech. After the Newtown shooting, Rodgaard said, “I no longer felt it was safe to raise a family in this country.”

Ed O’Keefe contributed to this report.

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