One of the key gun control measures backed by US President Barack Obama in the wake of the Sandy Hook school massacre has been dropped from proposed legislation.
Leader of the Democrat-controlled Senate Harry Reid said a proposal to ban assault weapons and large capacity magazines did not command enough support to pass.
It was the latest blow to the White House's gun control plans, which are fading as Republicans and even some Democrats baulk at taking on the powerful gun lobby.
Less than half of the Senate backs the assault weapons ban, Mr Reid said, which would condemn it to failure when gun control legislation comes to the floor of the chamber next month.
Prohibition on the sale of assault weapons was always the most controversial element of Mr Obama's attempt to stem gun violence since December's massacre at the Connecticut school where 26 people, including 20 children, died.
Backed by a lobbying campaign from the National Rifle Association, many politicians argue that bringing back a ban that ran out in 2004 infringes Americans' constitutional right to bear arms.
Mr Reid told reporters that the bill by Senator Dianne Feinstein to renew the ban currently "has less than 40 votes" in the 100-member chamber.
Other gun control efforts such as universal background checks on people buying guns are also struggling in Congress.
Along with immigration reform, gun control is a top domestic policy priority for Mr Obama at the start of his second term.
White House Chief of Staff Denis McDonough has refused to concede defeat on the assault weapons ban.
"We're going to work on this. We're going to find the votes, and it deserves a vote. Let's see if we can get it done," Mr McDonough told CNN.
Senators are likely to vote on the ban in April as an amendment to other gun control legislation, but its chances of success are now virtually nil.
Sixty votes would be needed to clear an anticipated Republican procedural roadblock.
Democrats control the chamber, 55-45, meaning that a number of Mr Reid's fellow Democrats have made it clear that they intend to oppose renewing the ban.
The ban on assault weapons is one of four gun control bills that the Democratic-led Judiciary Committee has sent to the full Senate.
A proposal to provide funding for school security is now the only gun violence measure sure to pass. Two others face an unclear future.
An effort to expand background checks on gun buyers has run into opposition from Republican politicians, who fear that could be a first step toward registering gun owners.
The other plan would make it a federal crime to purchase a firearm on behalf of someone who is barred from owning one.
However, critics of the measure complain that it is difficult for a gun seller to know who is allowed to buy a weapon and who is not.