The senators, Democrat and Republican, who killed the bipartisan compromise amendment on background checks for gun sales explained their opposition in terms of principle. But gun reformers will view it more as political cowardice on the part of politicians fearful of damaging their re-election chances.

The vote was on a proposal agreed between Democrat Joe Manchin and Republican Pat Toomey that would have required criminal and mental health background checks for advertised sales of guns, including at gun shows and online.

It was the last meaningful measure left in a draft bill that had already been stripped of most of its substance.

Vice-president Joe Biden, on his way to the Hill for the vote, said: "I hope to God that there's 60 people up there who have the courage to stand up." Less than an hour later he got his answer.

Here are the key votes that killed the measure, just four months after Newtown.

Heidi Heitkamp, 57, Democrat, North Dakota

Heitkamp has the distinction of being the first Democratic senator to break ranks by announcing publicly that she would vote against background checks.

Unlike some of her colleagues, she does not face the prospect of re-election next year, having just been elected in November. But that victory was extremely tight, with her winning by just 2,994 votes.

She has said repeatedly in interviews that guns are part of the way of life in North Dakota and her priority is the interests of her state. One of her campaign ads set out her platform as "schools and tractors and guns".

In the aftermath of the Newtown shooting, she dismissed Barack Obama's proposals for gun reform as "extreme".

Mark Begich, 51, Democrat, Alaska

Begich is up for re-election next year in a state that normally votes Republican. He was one of only two Democrats to vote last week against starting a debate on gun reform.

A large part of the Democratic psyche is conditioned to believe that their disastrous losses in the 1994 congressional elections can be blamed to a large extent on Bill Clinton's assault weapons ban.

Barack Obama called Begich seeking his support, but the senator argued that what was needed is not new legislation but enforcement of existing laws.

In 2008, Begich became the first Democrat to represent Alaska in the Senate since 1980 after the long-serving Republican senator Ted Stevens was found guilty just eight days before the election of seven counts of making misleading statements. The indictment was later thrown out over prosecutorial misconduct.

Begich, who faces a tough re-election battle next year, attributed his win in part to his opposition to gun control.

Mark Pryor, 50, Democrat, Arkansas

Pryor was the other Democrat who voted last week against bringing the bill to the floor of the Senate. Although representing a traditionally red state, he won convincingly in 2002 by 54% to 46%, partly because he is an evangelical Christian.

Pryor was easily re-elected six years later when the Republicans opted against fielding a candidate. He famously said in a documentary: "You don't need to pass an IQ test to be in the Senate".

Max Baucus, 71, Democrat, Montana

Facing re-election next year, Baucus is one of the Democrats scarred by the belief that gun reform in 1994 cost the party votes. A senator for 35 years, the closest Baucus came to defeat was that year, winning by a margin of five points.

He voted for the 1994 assault weapons ban, but is this time not repeating what he would view as that mistake. He is the only Senate Democrat with an A+ rating from the National Rifle Association.

Kelly Ayotte, 44, Republican, New Hampshire

Gun reformers rested their hopes on securing the support of Republicans such as Ayotte to offset the loss of the four Democrats. She was classified as a potential swing vote. Those hopes rose further when she was among 16 Republicans who voted last week to open the debate.

But, in a statement on Wednesday, she said: "While steps must be taken to improve the existing background check system, I will not support the Manchin-Toomey legislation, which I believe would place unnecessary burdens on law-abiding gun owners and allow for potential overreach by the federal government into private gun sales."

Lisa Murkowski, 55, Republican, Alaska

Like Ayotte, she is one of the few Republicans the gun-reformers hoped might be persuadable. But she announced on Tuesday night she was resisting all attempts to win her round and would vote against expanded background checks.

Unlike others who had been sitting on the fence but eventually voted against, she is in a relatively strong position electorally. Her popularity in Alaska was confirmed in 2010 when she lost a Republican primary to the Tea Party-backed Joe Miller but won re-election with a write-in campaign.

Manchin and Toomey altered their plan to try to win her over, putting in an exception to background checks for those living in remote areas far from gun stores.

It failed to sway her. In a statement, Murkowski wrote that Alaskans "want to keep Americans safe as much as anyone, but don't believe they need to compromise their second amendment rights to get there".

Dean Heller, 52, Republican, Nevada

Heller announced on Tuesday he would vote against the amendment.

"Despite the good faith efforts of senators Manchin and Toomey, the onerous paperwork and expansion of federal power mandated in this legislation are too great of a concern. I believe that this legislation could lead to the creation of a national gun registry and puts additional burdens on law-abiding citizens," he said.