The Senate on Wednesday defeated a vital background check amendment seen as the linchpin to Democrats' gun control bill, dealing a major setback to President Obama -- who lashed out at opponents in unusually blunt terms during remarks from the Rose Garden.

"All in all, this was a pretty shameful day for Washington," Obama said, accusing the gun lobby of lying about the bill.

The vote was 54-46, with supporters falling six votes short of the required 60-vote threshold.

The failure of the background check proposal authored by Sens. Joe Manchin, D-W.Va., and Pat Toomey, R-Pa., now imperils the entire legislation. The proposal would have expanded background checks to gun shows and Internet sales while exempting personal transactions. The amendment was aimed at winning over reluctant conservatives, who were opposed to the more stringent background check plan in the existing bill.

It's unclear where supporters will go from here. They could try to vote again, or craft an alternative piece of legislation.

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Obama vowed to press on, saying the vote was "just round one," while decrying those he claimed "caved" to political pressure.

"The gun lobby and its allies willfully lied about the bill," Obama said. He said the claims "upset" some gun owners who in turn "intimidated" senators.

"There were no coherent arguments as to why we couldn't do this. It came down to politics," he said.

In a statement, Manchin said that while he is disappointed in the outcome of today's vote, that "this is not the end of the debate."

Opponents, which included a few Democrats, voiced concern that the proposal would infringe on Second Amendment rights by imposing a burden on those buying and selling guns. They claimed the proposed system would not have prevented Newtown, and would not stop criminals. They also voiced concern about the possibility that the expanded system could lead to a gun registry, though the amendment language prohibits this.

The NRA said in a statement that the 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."

But Senate Majority Leader Harry Reid and other Democrats hammered Republicans for not voting in support and vowed to press forward.

"I want everyone to understand this is just the beginning, not the end," said Reid. "Today, the brand of the Republican Party has become more out of step, more extreme. And that's saying something."

Four Republicans voted for the amendment, but five Democrats voted against it. One of those Democrats was Reid -- who only switched his vote to oppose it because doing so allows Democrats to call up the measure again. Other Democrats who voted against the measure for non-procedural reasons were Sen. Mark Pryor of Arkansas, Sen. Mark Begich of Alaska, Sen. Heidi Heitkamp of North Dakota and Sen. Max Baucus of Montana.

The Obama administration has made the package, written in the wake of the Newtown school mass shooting, a top priority and along with its allies had applied heavy pressure to wavering lawmakers. Vice President Biden presided over the vote Wednesday.

Lawmakers proceeded to vote on a string of other amendments Wednesday, including a proposed ban on assault weapons and a ban on high-capacity magazines, which were defeated as well.

In the run-up to Wednesday's vote, Democratic leaders gave ever-changing assessments of where support stood.

Biden said Tuesday that Democrats would get the 60 votes, but then said later in the day that it could come down to one or two senators.

Manchin acknowledged early Wednesday that the bill was having trouble, but then released a statement saying he remained "optimistic and hopeful."

Opponents needed just 41 of the Senate's 100 votes to derail the Manchin-Toomey background check plan.

Thirty-one senators voted last week to completely block debate on overall gun legislation. Since last week, enough lawmakers who voted to allow debate switched to oppose Manchin-Toomey, in turn defeating the amendment.

"I believe very strongly that our current background check system needs strengthening and improving, particularly in areas that could keep guns out of the hands of felons and the mentally ill. At the same time, I cannot support legislation that infringes upon the constitutional right to keep and bear arms," Sen. Dean Heller, R-Nev., one of those opposed, said in a statement.

Sen. Mike Lee, R-Utah, echoed Heller in a statement released following the vote saying "Following the tragedy at Sandy Hook, we all wanted to find answers that would reduce crime and prevent the next senseless act of violence. Unfortunately, the Senate did not consider any proposals that would achieve these objectives."

Lee also said he agreed with the president that the debate was not over and hoped that they could "discuss the problems that lead to these violent acts" and work on solutions that actually address them.

Only four Republican senators committed to voting for the amendment ahead of time. The last was Sen. John McCain, R-Ariz., who announced his support Wednesday afternoon. The other three were Toomey, Sen. Mark Kirk of Illinois and Sen. Susan Collins of Maine.

The Senate gun bill would extend background checks to nearly all gun purchases, toughen penalties against illegal gun trafficking and add small sums to school safety programs.

Perhaps helping explain Democrats' problems, an AP-GfK poll this month showed that 49 percent of Americans support stricter gun laws. That was down from 58 percent who said so in January -- a month after the December killings of 20 children and six aides at a Newtown, Conn., elementary school propelled gun violence into a national issue. Family members of Newtown victims, some tearing up after the vote, also criticized the Senate for the amendment's failure Wednesday.

The Senate held eight other votes Wednesday besides the one on background checks, all of them amendments to the broad gun control measure.

They included Democratic proposals to ban assault weapons and high-capacity ammunition magazines, which are expected to lose; a Republican proposal requiring states to honor other states' permits allowing concealed weapons, which faces a close vote; and a GOP substitute for the overall gun measure. The concealed weapons amendment, seen by advocates as protecting gun rights, was vehemently opposed by gun control groups, who say it would allow more guns into states with stricter firearms laws.

The other amendments were defeated.

The votes were coming a day after former Rep. Gabrielle Giffords, badly injured in a 2011 mass shooting in Tucson, Ariz., and her husband, Mark Kelly, tried galvanizing gun control support by visiting Capitol Hill and attending a private lunch with Democratic senators. Reid, D-Nev., called the lunch -- senators said it included emotional speeches from lawmakers -- "as moving as any" he has attended.

The Associated Press contributed to this report.