President Obama accused members of Congress of having "a pretty shameful day in Washington," a reaction to the Senate's failure to pass a key gun control measure that would have expanded background checks.
The proposal to expand the background checks for people buying guns online and at gun shows fell six votes short of winning the 60 votes needed to pass. The vote was 54 to 46.
Obama, introduced by Mark Barden, whose 7-year-old son, Daniel, was killed at Sandy Hook Elementary in Newtown, Conn., made his displeasure clear in his tone and grim facial expression as he scolded lawmakers and assured constituents that the vote was just "round one."
"Most of these senators could not offer any good reason for why we wouldn't want to make it harder for criminals and those with severe mental illnesses to buy a gun," Obama said.
The president admonished senators who, in his words, "caved to the pressure" from a powerful gun lobby.
"I've heard some say that blocking this step would be a victory," Obama said. "My question is: A victory for who? A victory for what? All that happened today was the preservation of the loophole that lets dangerous criminals buy guns without a background check."
The president called for sustained public passion behind gun control, and he called on supporters of background checks to call their lawmakers, register their disappointment, and vote gun-control opponents out of office unless they change.
"When Newtown happened, I met with these families, and I spoke to the community and I said something must be different," Obama said.
"That's what the whole country said," he added. "Everybody talked about how we were gonna change something to make sure this didn't happen again, just like everybody talked about how we were gonna have to do something after Aurora, just like everybody talked about how we were gonna have to change something after Tucson. ... I'm assuming our expressions of grief and our commitment to do something different to prevent these things from happening are not empty words."
In addition to the Newtown families, President Obama was joined by former Arizona Rep. Gabrielle Giffords, who survived an attempted assassination in 2011, and Vice President Joe 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 failure of the measure on background checks, backed by Sens. Joe Manchin, D-W.Va., and Pat Toomey, R-Pa., was a striking defeat for President Obama and supporters of gun control in the wake of the Sandy Hook shooting.
Several family members of the Sandy Hook massacre also were seated in the Senate gallery at the time of the failed vote. As the Senate called vote, Lori Haas, the mother of a woman who survived being shot twice at Virginia Tech University, and Tucson survivor Pat Maiach yelled, "Shame on you!" Their shouts were audible from the Senate floor. They then were detained inside the Capitol building, being interviewed by Capitol Hill police.
As anticipated, the Senate also failed to pass the assault weapons ban, by a vote of 40-60.
Biden, who was presiding over the Senate and announced the vote on background checks, reacted with scorn, saying: "This is far from over. This is far from over."
"The United States Senate let down an awful lot of people today, including those Newtown families," Biden told ABC News as he left the Capitol. "I don't know how anybody who looked them in the eye could have vote the way they did today. You know, it's time for the American people to make it clear how displeased they are with this vote and let their representatives know that."
The National Rifle Association issued a statement calling the defeated amendment "misguided."
"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," NRA spokesman Chris Cox said in a statement.
"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," he said.
New York Mayor Michael Bloomberg, however, issued an angry statement on behalf of Mayors Against Illegal Guns.
"Today's vote is a damning indictment of the stranglehold that special interests have on Washington," the group's statement said. "More than 40 U.S. senators would rather turn their backs on the 90 percent of Americans who support comprehensive background checks than buck the increasingly extremist wing of the gun lobby."
Gov. Dan Malloy of Connecticut, which has passed some of the strictest gun laws in America in the past month, questioned why senators would vote against a measure he believed to be very popular with the public.
"There is much more that needs to be done on the issue of gun violence prevention. But when the Senate cannot come together on an issue that is supported by the vast majority of Americans, there is little to no hope that common sense will prevail," Malloy said in a statement. "The members who voted against this proposal should be ashamed of themselves."
Shortly after the vote, the Progressive Change Campaign Committee announced it would launch an ad campaign targeting Democratic senators who voted against the bill. Those included Mark Pryor, D-Ariz., Mark Begich, D-Alaska, Max Baucus, D-Mont. and Heidi Heitkamp, D-N.D.
Earlier today Manchin told ABC News that regardless of the vote's outcome, the need to impose new gun measures "would not go away."
ABC News' Mary Bruce contributed to this report.