Despite an impassioned push by President Barack Obama and an emotional lobbying effort by the families of mass shooting victims, proponents of a compromise measure to expand gun background checks on Wednesday fell six votes short of passage in the Senate.
The vote on the amendment was 54 to 46. Sixty votes were needed for the amendment to be adopted.
The deal was the result of a deal struck between Republican Pat Toomey of Pennsylvania and Democrat Joe Manchin of West Virginia. It would have extended existing background check rules to gun sales made online and at gun shows.
Speaking in the Rose Garden after the vote, a visibly frustrated Obama decried the defeat of the measure as parents of victims of last year's Newtown school shootings and former congresswoman Gabby Giffords looked on.
"All in all, this was a pretty shameful day for Washington," he said sternly, urging backers of gun control to continue the fight.
Slamming critics who said that victims were used as "props," Obama said that their voices and experiences should have been welcomed, adding that gun lobbyists "willfully lied" about the consequences of the background check measure.
As the bill was defeated, Patricia Maisch -- a survivor of the Tuscon shooting that targeted Giffords -- yelled "Shame on you!" from the Senate gallery.
After the vote, Maisch said outside the chamber that she screamed when she realized the amendment had been defeated.
"They need to be ashamed of themselves," she said. "I think the ones who voted no ... they have no soul. They have no compassion for the experiences that people have lived through, gun violence, who have had a child or a loved one murdered."
Although backed by many victims of gun violence, including the Newtown families, the legislation was vehemently opposed by the National Rifle Association, who said it infringed on the rights of gun owners.
In a statement, NRA-ILA executive director Chris Cox applauded the proposal's defeat: "This amendment would have criminalized certain private transfers of firearms between honest citizens, requiring lifelong friends, neighbors and some family members to get federal government permission to exercise a fundamental right or face prosecution," he said. "As we have noted previously, expanding background checks, at gun shows or elsewhere, will not reduce violent crime or keep our kids safe in their schools."
Four Republicans -- Sens. John McCain of Arizona, Susan Collins of Maine, Mark Kirk of Illinois and amendment author Toomey -- broke with the rest of the GOP to support the background check legislation.
Four Democrats -- Sens. Max Baucus of Montana, Mark Begich of Alaska, Heidi Heitkamp of North Dakota and Mark Pryor of Arkansas -- voted against it.
Those Democrats are now the targets of liberal groups vowing to advertise their 'no' votes. In a statement shortly after the defeat, Progressive Change Campaign Committee co-founder Stephanie Taylor slammed the four defectors.
"Today, the Senate voted against the 91% of Americans who support background checks to stop gun violence," she said. "We'll be holding accountable Democrats who voted against their constituents by running ads in their states, featuring some of the 23,000 gun owners who have joined our campaign for common sense gun reform."
Also on Wednesday, the Senate voted down an amendment backed by Sen. Dianne Feinstein of California that would have banned "military-style" assault weapons; it was voted down 40 to 60. By a similar margin, a proposal to limit high-capacity ammunition magazines was also defeated.
Opposition to changing the gun laws in previous weeks was so intense that some questioned whether the background check measure would even come to a vote Wednesday. Gun control backers won a surprisingly robust bipartisan victory on a procedural vote last week that allowed debate on the background check deal.
Much of the momentum that fueled that brief victory was credited to the parents and relatives of children killed in the Newtown shootings last year. Newtown families lobbied extensively on the Hill, reportedly bringing several lawmakers to tears with deeply personal stories of the grief caused to their families when a lone gunman mowed down 20 children and six adults at Sandy Hook Elementary School. Some of the Newtown family members were present in the Senate gallery when the background check amendment was defeated.
Giffords, who narrowly escaped death after being shot in the head in 2011, and her husband Mark Kelly slammed senators for "ignoring the will of the American people."
"We will use every means possible to make sure the constituents of these senators know that their elected representatives ignored them, and put Washington, D.C., special interest politics over the effort to keep their own communities safer from the tragedy of gun violence," they said in a statement.
Vice President Joe Biden, who has led the White House’s effort on the gun legislation after the Newtown shooting, presided over the Senate for the vote.
Speaking during a White House-organized Google Hangout earlier Wednesday, Biden appeared to acknowledge that defeat was likely but assured supporters that victory was not lost forever.
“If we don't get it today, we'll get it eventually,” he assured gun control supporters.
"I see this as just Round One," Obama said in his Rose Garden remarks.
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NBC's Mark Murray contributed to this report.