Standing alongside tearful families of the victims of the Sandy Hook massacre, a seething-mad President Obama lashed out against lawmakers who opposed a bill that would have expanded background checks for gun buyers, saying today marked a "shameful day for Washington."

"There were no coherent arguments as to why we wouldn't do this. It came down to politics," the president said in a Rose Garden statement shortly after the Senate defeated the bipartisan Manchin-Toomey amendment.

The president said lawmakers who opposed the measure "caved" to pressure from the gun lobby and its allies who "willfully lied about the bill."

"A minority in the United States Senate decided it wasn't worth it," he said. "They worried that that vocal minority of gun-owners would come after them in future elections. They worried that the gun lobby would spend a lot of money and paint them as anti-Second Amendment. And obviously a lot of Republicans had that fear, but Democrats had that fear, too. And so they caved to the pressure, and they started looking for an excuse, any excuse, to vote no."

In addition to the Newtown families, the president was joined by former Arizona Rep. Gabrielle Giffords, who survived an attempted assassination in 2011, and Vice President Biden, who has led the administration's effort to reform the nation's gun laws in the wake of the December shooting at Sandy Hook Elementary School.

The president vehemently pushed back against Sen. Rand Paul's claim that the White House used the Newtown families, who have spent weeks lobbying lawmakers, as "props" in the gun control debate.

"Do we really think that thousands of families whose lives have been shattered by gun violence don't have a right to weigh in on this issue? Do we think their - their emotions, their loss is not relevant to this debate?" Obama asked.

The president vowed "this effort is not over" and urged Americans to voice their opposition to inaction.

"I see this as just round one," he said. "I believe we're going to be able to get this one. Sooner or later we are going to get this right. The memories of these children demand it, and so do the American people."