That last vote marked a rare defeat for the National Rifle Association on a day it generally emerged triumphant over President Barack Obama, gun control advocates and individuals whose lives have been affected by mass shootings in Connecticut and elsewhere, some of whom watched from the spectator galleries above the Senate floor.
"Shame on you," shouted one of them, Patricia Maisch, who was present two years ago when a gunman in Tucson, Ariz., killed six and wounded 13 others, including former Rep. Gabrielle Giffords.
Vice President Joe Biden gaveled the Senate back into order after the breach of decorum.
The background check measure commanded a majority of senators, 54-46, but that was well short of the 60 votes needed to advance. Forty-one Republicans and five Democrats sided to scuttle the plan.
In the hours before the key vote, Sen. Joe Manchin, D-W.Va., bluntly accused the National Rifle Association of making false claims about the expansion of background checks that he and Sen. Pat Toomey, R-Pa., were backing.
"Where I come from in West Virginia, I don't know how to put the words any plainer than this: That is a lie. That is simply a lie," he said, accusing the organization of telling its supporters that friends, neighbors and some family members would need federal permission to transfer ownership of firearms to one another.
The NRA did not respond immediately to the charge, but issued a statement after the vote that restated the claim. The proposal "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," said a statement from Chris Cox, a top lobbyist for the group.
Said Sen. Chuck Grassley, R-Iowa, "Expanded background checks would not have prevented Newtown. Criminals do not submit to background checks."
The events were unlikely to be the last word on an issue that Democratic leaders shied away from for nearly two decades until Obama picked up on it after the Newtown shootings.
Vice President Joe Biden presided over the Senate, a symbolic move since each proposal required a 60-vote majority to pass and he would not be called upon to break any ties. Democratic aides said in advance the issue would be brought back to the Senate in the future, giving gun control supporters more time to win over converts to change the outcome.
The day's key test concerned the background checks, designed to prevent criminals and the seriously mentally ill from purchasing firearms. Under current law, checks are required only when guns are purchased from federally licensed firearms dealers. The proposal by Manchin and Toomey called for extending the requirement to other sales at gun shows and on the Internet.
Their bipartisan approach was widely seen as advocates' best chance for winning enough GOP votes to change current law in a way that Obama and gun control groups support. But foes had proposals of their own, including one that would require states that issue concealed weapons permits to honor the permits from other states.
In the hours leading to a vote on the background check measure, Democrats Heidi Heitkamp of North Dakota and Mark Pryor of Arkansas and Republican Kelly Ayotte of New Hampshire became the latest senators to announce their opposition.
On the vote, Democratic Sens. Mark Begich of Alaska and Max Baucus of Montana joined Pryor and Heitkamp in voting against the proposal. Senate Majority Leader Harry Reid, a supporter of the plan, switched his vote to the prevailing "no" side to permit him to call for a revote in the future.
Begich, Pryor and Baucus are all seeking re-election next year. In an indication of the intensity of the feelings on the issue, the Progressive Change Campaign Committee, a liberal group, swiftly announced it would seek to defeat them in 2014.
Among Republicans, Sens. Susan Collins of Maine, Mark Kirk of Illinois, John McCain of Arizona and Toomey sided with Democrats.
Numerous polls in recent months have shown support for enhanced gun control measures, including background checks, though it may be weakening.
An Associated Press-GfK poll this month showed that 49 percent of Americans support stricter gun laws, down from 58 percent in January. In that recent survey, 38 percent said they want the laws to remain the same and 10 percent want them eased.
Obama has made enactment of greater curbs a priority on his domestic agenda in the months since the massacre at Newtown, making several trips outside Washington to try and build support. Last week, he traveled to Connecticut, and he invited several parents to fly back to Washington with him aboard Air Force One so they could personally lobby lawmakers.
To an unusual degree for professional politicians, some senators said afterward that they had not wanted to meet with the mothers and fathers of the dead, or said it was difficult to look at photographs that the parents carried of their young children, now dead.
Sen. Rand Paul, R-Ky., said before Wednesday's vote, "I think that in some cases, the president has used them as props, and that disappoints me."
Some of the parents of the Sandy Hook victims watched the votes from the spectators' gallery that rings the Senate floor. They were joined by relatives of victims of other mass shootings in Arizona, Virginia and Colorado.
At the White House, press secretary Jay Carney said some of them had met earlier in the day with lawmakers, who he said should "consider who they're representing.
"Ninety percent of the American people support expanded background checks," he said.
Democratic aides said in advance that the day's events would end debate on the issue for the time being. But they added they expect Senate Majority Leader Harry Reid to bring it back to the floor in the coming months, after supporters of greater controls have had more time to rally public support.
The NRA told lawmakers it intended to keep track of how the votes were cast, and consider them in making decisions about its efforts in the mid-term elections for Congress next year.
An opposing group, Mayors Against Illegal Guns, funded by New York City Mayor Michael Bloomberg, said it would do likewise.
The NRA has a long track record in electoral politics, and is viewed by lawmakers in both political parties as unusually effective. Bloomberg's organization has yet to be tested.
The day began with an unexpected announcement from Reid, who has long been a political ally of the NRA. In remarks on the Senate floor, he said he intended to vote for a ban on assault weapons "because maintaining law and order is more important than satisfying conspiracy theorists who believe in black helicopters."
In the AP-GfK poll, among independents, support for stricter gun laws dipped from 60 percent in January to 40 percent now. About three-fourths of Democrats supported them then and now, while backing among Republicans for looser laws about doubled to 19 percent.
The survey was conducted from April 11-15 by GfK Roper Public Affairs and Corporate Communications. It involved landline and cellphone interviews with 1,004 randomly chosen adults and had a margin of sampling error of plus or minus 3.9 percentage points.
Senate kills Dems' attempt to ban assault weapons
The Senate has rejected an attempt by Democrats to ban assault weapons, one of their leading answers to December's slayings of children and staff at an elementary in Newtown, Conn.
Led by Democratic Sen. Dianne Feinstein of California, supporters said barring the military-style weapons would reduce the deadliness of gun crimes because shooters wouldn't be able to fire as many shots.
But Republicans and many Democrats opposed taking a step they say would curtail the Second Amendment right to bear arms. They also argued that prohibiting the weapons would do little because assault weapons account for a small portion of gun crimes.
The defeat Wednesday was expected.
The plan was rejected 60-40.
Obama slams senators who opposed gun measure
President Barack Obama says the Senate's opposition to a bill that would have expanded background checks for gun buyers marks a "shameful day" in Washington. He says a minority of senators decided "it wasn't worth it" to protect the nation's children.
Obama spoke in the Rose Garden shortly after the Senate vote. It marked a major blow to the gun control push Obama started in the wake of December's shooting at a Newtown, Conn., elementary school.
The president pinned the blame for the measures failure, though five Democrats also opposed the plan.
Obama was introduced by the father of a 7-year-old killed in the shooting. Other families joined him in the Rose Garden, along with former Arizona Rep. Gabrielle Giffords, who was shot in the head in 2011.