A compromise proposal to expand background checks on gun sales lacks enough support to win Senate approval now, which will likely delay a vote on the measure that has strong public support, one of the sponsors told CNN on Monday.
Sen. Joe Manchin, D-West Virginia, acknowledged the vote on the amendment he worked out with Republican Sen. Pat Toomey of Pennsylvania was being pushed back to try to build more support.
Asked if the vote would take place Tuesday or Wednesday, as supporters had hoped, Manchin said he didn't think so.
"I would say by the end of the week, probably," he added.
The Manchin-Toomey compromise had been scheduled as the first amendment to be considered as part of gun legislation pushed by President Barack Obama and Democrats in the aftermath of the Newtown school massacre in December.
Last week, the broader gun package received 68 votes, including 16 from Republicans, to bring it to the Senate floor.
Before discussion on the legislation began Monday, Manchin told CNN there was more work to do.
"We've got to work hard and the more people know about the bill, the more people read the bill, the more people see the facts of the bill, it breaks down all the misnomers," he said of what he called misinformation about the background check proposal being spread by opponents.
Both Manchin and Toomey are self-professed gun lovers who have gotten high marks from the powerful National Rifle Association for their voting records on firearms issues.
However, the NRA opposes their compromise, casting doubt on whether it can get the 60 Senate votes necessary to pass under the filibuster threshold imposed by Republicans.
Far fewer Republicans than the 16 who voted last week to open debate are committed to backing the Manchin-Toomey amendment or supporting the final gun package.
So far, four GOP senators have declared they will support the Manchin-Toomey compromise or were leaning toward backing it.
With at least two moderate Democrats from pro-gun states known to oppose the measure, it needs at least seven GOP votes to have any chance of passing.
The amendment will be one of perhaps dozens to be debated on the broader package of gun laws drafted in response to the killing of 20 first-graders and six educators at the Sandy Hook Elementary School in Newtown, Connecticut.
The package also contains measures to crack down on gun trafficking and straw purchases, and to find ways to improve school safety.
On Monday, White House spokesman Jay Carney sought to keep up the pressure on senators to approve the expanded background checks, noting polls show about 90% of Americans support such a move.
"If they vote no, they will be siding with the 10% and not the 90% in the United States that support background check legislation," Carney told reporters.
In debate expected to last two weeks, senators will consider efforts by both sides to either expand or weaken the gun package.
A push by Democrats for an updated ban on semiautomatic firearms modeled after military assault weapons, as well as limiting ammunition magazines to 10 rounds, is expected to fail.
Republicans will push an NRA proposal to make state concealed weapons permits acceptable nationwide, a concept opposed by many Democrats.
Since the Newtown attack by a lone gunman, Connecticut and a few other states have passed tougher gun laws.
While polls show Americans favoring expanded background checks and other proposals intended to crack down on gun trafficking, more pro-gun states have passed laws easing firearms restrictions.
In addition, the NRA has promised political retribution for legislators who back measures it opposes.
"This debate in some ways is underscoring just the extent to which there is a polarization in our society," Toomey told CNN on Sunday, lamenting "the acrimony that has gotten into politics."
Asked if he was concerned about political repercussions, the conservative Republican said: "I'll just let the political chips fall the way they fall."