A deeply emotional Obama lashed out after a bid to expand background checks for gun buyers -- the last meaningful piece of his trimmed effort to retool gun laws after the Newtown school massacre -- failed to pass.
Ringed by dazed and grief-stricken relatives of some of the 20 children gunned down in Connecticut in December, Obama accused the firearms lobby of lying to thwart change but vowed he would not give up the fight.
"Families that know unspeakable grief summoned the courage to petition their elected leaders not just to honor the memory of their children but to protect the lives of all of our children," he said in the White House Rose Garden.
"A minority in the United States Senate decided it wasn't worth it."
As he absorbed the first significant political defeat of his second term, the president said "the gun lobby and its allies willfully lied about the bill.
"All in all, this was a pretty shameful day for Washington."
Obama was clearly furious about the vote, months after vowing to use all his power to enact gun reform in the tearful aftermath of the Newtown tragedy.
In a scathing rebuke, Obama complained that Republicans and some of his own Democrats had simply been scared of the well-financed gun lobby.
"They caved to the pressure. And they started looking for an excuse, any excuse, to vote no," he said, vowing to fight on for change.
"I see this as just round one," the president added, hinting that he would campaign on the issue in the 2014 mid-term elections.
"I believe we're going to be able to get this done. Sooner or later we're going to get this right. The memories of these children demand it."
Republicans argued that the bill infringed on the constitutional right to bear arms, but Obama said it did no such thing and merely tried to stop criminals and the mentally ill from getting firearms.
Some conservative Democrats, fearing a backlash as they run for re-election, also opposed the bill.
Chris Cox, chief lobbyist for the National Rifle Association, the top pro-gun pressure group, said the bill was "misguided" and would have criminalized arms transfers between law-abiding citizens and friends.
"Expanding background checks, at gun shows or elsewhere, will not reduce violent crime or keep our kids safe in their schools," Cox said.
Obama's calls for a reinstated ban on assault weapons and high-capacity magazine clips also crashed in the Senate, as expected.
The background checks bill did attract a majority vote -- 54-46 -- but 60 senators were needed for passage.
After the measure went down, one gun violence survivor, Patricia Maisch, shouted "Shame on you!" from the Senate gallery.
Maisch was at the 2011 shooting that left six people dead and then-congresswoman Gabrielle Giffords gravely wounded in Arizona.
Giffords was at Obama's side in the White House Rose Garden, along with vice president Joe Biden and traumatized parents of several Newtown children.
"They have no souls, they have no compassion for the experiences that people have lived through (with) gun violence, who have had a child or a loved one murdered by a gun," Maisch said.
Obama was incensed by claims from some conservatives that he had used relatives of victims as political props in the campaign debate.
"Do we really think thousands of families whose lives have been shattered by gun violence don't have a right to weigh in on this issue?" he asked.
Before Obama spoke, Mark Barden, whose seven-year-old son Daniel died at Sandy Hook Elementary School in Newtown, promised to carry on the battle.
"We will not be defeated and we are not defeated. We are not going away," he said, fighting back tears.
Shortly before the vote, one Republican Senator, John McCain, announced his backing, but he knew the amendment was already doomed.
"You did the right thing," he told his colleague Joe Manchin, who helped write the background checks bill.
McCain forecast that "sooner or later, this country will take up this issue."
Polls show that 90 percent of Americans back closing the loophole that allows people to purchase firearms at gun shows with no background checks.
Gun reform campaigners were devastated by the vote.
Josh Horwitz, director of the Coalition to Stop Gun Violence, said the vote was a "stain on the reputation of the US Senate."
Senators who opposed the bill "should be ashamed of themselves," said Connecticut Governor Dannel Malloy.
Democratic Senate Majority Leader Harry Reid also signaled an electoral effort on guns.
"The brand of the Republican Party has become more out of step, more extreme," Reid said.