Barring a last-minute change of heart, the chance for historic US gun legislation appeared to slip away Wednesday as most Republicans balked at supporting an expansion of background checks.
Four months after the shock massacre at an elementary school in Newtown, Connecticut, and amid persistent lobbying by President Barack Obama, an author of a compromise deal that required background checks for all firearm sales at gun shows and on the Internet acknowledged the bid was falling short.
The US Senate was set to vote on the measure at 4:00 pm (2000 GMT), followed by votes on up to eight more amendments -- introduced by Democrats as well as Republicans -- to underlying gun safety legislation.
Sixty votes were required to move the amendments forward and ensure final passage in the 100-member chamber, but by early Wednesday, after days of pressing his fellow lawmakers, Senate Democrat Joe Manchin acknowledged to NBC News that his bill "will not get the votes today."
And in an interview Wednesday with the National Review, the measure's Republican co-author, Senator Pat Toomey, said "as we sit here this morning, we don't have the votes."
The measure's defeat would mark a stinging defeat for Obama, who has expended considerable political capital on the issue since proposing several gun safety measures -- at the core of which was a call for expanding background checks -- in the wake of the Newtown shooting deaths in December.
But the White House refused to concede defeat.
"This isn't over. I'm not going to give a postmortem," spokesman Jay Carney told reporters.
"There is no question that the path to 60 in this case is difficult, but it is not unachievable.
"If you are opposed to this legislation, you should explain why you are against something that 90 percent of the people are for, that vast majorities of the people in your state support."
White House sources said the president had been calling wavering senators to push for support for the background checks bill in the hours before the vote and Carney said everyone in the White House from Obama on down was involved.
But talk on the floor suggested the deal was dead -- at least for now.
"You did the right thing," Republican John McCain told Manchin on the Senate floor, commending his colleague for his commitment to a politically heavy lift.
"Sooner or later, this country will take up this issue," McCain added.
Senate Majority Leader Harry Reid said he would allow votes on several amendments, including one that bans assault weapons, another which restricts the size of ammunition clips, a proposal that expands concealed-carry rights for gun owners, and a measure that funds expanded mental health programs.
A Republican-drafted measure, that "safeguards the Second Amendment" right to bear arms by expanding funding for federal gun prosecutions and offering incentives for states to provide mental health records to the federal background check database, will also receive a vote.
Reid lashed out at what he called the urge to satisfy "conspiracy theories," and said he would vote for Senator Dianne Feinstein's assault weapons ban "because saving the lives of police officers young and old, and innocent civilians young and old, is more important than preventing imagined tyranny."
Polls show an overwhelming majority of Americans support closing the loophole that allows people to purchase firearms at gun shows with no background check, but the powerful gun lobby, led by the National Rifle Association, is vehemently opposed.
A handful of Democrats were set to vote against the measure, including Mark Begich of Alaska and Mark Pryor of Arkansas, who both face tough 2014 re-election fights in conservative-leaning states.