The US Senate has rejected a plan to expand background checks for gun buyers, dealing a crippling blow to President Barack Obama's campaign to curb gun violence.

The plan to extend background checks to online and gun-show sales failed on a 54-46 vote, six short of the 60-vote hurdle needed to clear the Senate.

"All in all, this was a pretty shameful day for Washington," an angry Mr Obama said of the vote, adding the effort "is not over".

The amendment by Democrat Joe Manchin of West Virginia and Republican Pat Toomey of Pennsylvania had represented Mr Obama's best hope to pass meaningful gun-control legislation.

"Our hearts are broken. Our spirit is not," Mark Barden, the father of a victim of the Newtown shootings, said at the White House after the vote, with Mr Obama looking on.

"We always knew this would be a long road. We don't have the luxury of turning back."

Other measures backed by Mr Obama - including a proposal to ban rapid-firing "assault" weapons like the one used in Connecticut - also failed in a series of Senate votes that starkly showed the lingering political power of gun rights defenders and the National Rifle Association.

"It came down to politics," Mr Obama said, adding too many senators had worried a vocal minority of gun owners would come after them in the next election.

The votes were the culmination of weeks of intense negotiations and lobbying over Mr Obama's proposed gun restrictions.

The defeat of the background checks amendment could doom the biggest package of gun legislation Congress has considered in two decades.

"Shame on you!" a spectator in the gallery shouted as the tally was announced by Vice President Joe Biden, who presided over the Senate votes.

Political momentum for new gun-control laws had dissipated after December's shooting.

Opponents criticised the proposals as government overreach that would infringe on the constitutional right to bear arms, and the NRA mounted a strong lobbying effort against it.

The Manchin-Toomey background checks amendment allowed exemptions for private sales or gifts between families and friends and prohibited the creation of a national registry of guns.

But the NRA had warned members the proposal would require checks for sales and gifts between family and friends and lead to a national registry.

Polls show more than 80% of Americans support expanded background checks.