The Senate's top Democrat said Tuesday he will force a vote this week on whether to open debate on tougher gun laws, increasing pressure on legislators from both parties negotiating a possible compromise on a package that some Republicans have threatened to filibuster.
A GOP filibuster would mean Senate Majority Leader Harry Reid needs 60 votes on Thursday just to begin Senate consideration of the package based on proposals by President Barack Obama in the aftermath of the Newtown school massacre in December that killed 20 first-graders and six educators.
Obama has made gun measures a major focus of his second term agenda, holding events across the country to push for Congress to vote on the package.
He spoke Monday in Connecticut, the state where the Newtown shootings occurred, and Vice President Joe Biden made a similar call for action at the White House on Tuesday.
Reid told reporters he hoped to get a bipartisan deal before the procedural vote on Thursday.
Talks involving Democratic Sen. Joe Manchin and Republican Sen. Pat Toomey of Pennsylvania have focused on a compromise on expanding background checks of gun buyers.
Manchin indicated late on Tuesday the two were close to an agreement and have scheduled a news conference for Wednesday morning. A Manchin aide told CNN they are "hopeful" to have a deal to announce.
Separately, Democratic leaders appear increasingly optimistic they will get enough Republican votes to overcome a threatened filibuster on starting debate.
Democrats believe as many as a dozen Republicans will side with them and that will more than make up for the handful of pro-gun Democrats who might not.
At least five Republicans have publicly opposed the filibuster pledged by 14 of their Senate colleagues, including GOP leader Sen. Mitch McConnell of Kentucky.
McConnell later told reporters his filibuster threat referred to the specific package approved by the Senate Judiciary Committee and indicated he might be open to a compromise.
Sen. Kelly Ayotte, R-New Hampshire, said Tuesday she favored opening debate on the bill as long as amendments can be offered, adding: "I think it's an important debate to have and I do not believe we should block the motion to proceed."
Without a deal, a successful Republican filibuster would stop consideration of the gun legislation before any votes on specific provisions.
Democratic leaders want to give senators from both parties ample opportunity to amend the bill and are prepared to debate it beyond a scheduled recess the first week of May, if doing so will increase chances of passage.
"The way you put together a coalition to pass the bill is to allow as many amendment votes as you can. We are willing to take the time to do that and have that process," a Democratic leadership aide told CNN.
Obama's rhetoric has reflected the political uncertainty, with the president and his aides using increasingly personal language intended to shame Republicans into allowing public votes on measures that have public support but are fiercely opposed by the influential National Rifle Association.
"If senators don't have the guts to go on the record to vote how they feel on this issue ... that would be a shame and that would be a disservice to their constituents," White House spokesman Jay Carney told reporters on Tuesday.
At a later White House event intended to keep up the public profile of the issue, Biden said Republican efforts to block tougher gun laws showed they were in a "time warp" because public support on issues such as expanded background checks "has moved beyond where it was five, ten, even three years ago."
On the other side, the NRA and its supporters in Congress argue the Democratic proposals threaten the constitutional right to bear arms, and also offer ineffective responses intended as political show instead of real solutions to the problem of gun violence in America.
"On firearms questions, on Second Amendment questions, there's a divide in this country," NRA President David Keene told CNN. "To call it an ideological divide is too simple because it's a cultural divide. When something happens, the folks on the other side from us say, 'well the problem's the gun, we need to do something about guns.'"
Defeat of the entire package in the Democratic-led Senate would kill gun legislation for now, which would be a stinging defeat for Obama and Democrats pushing for tougher laws.
However, a public perception that Republicans blocked popular proposals such as expanding background checks of gun buyers could harm GOP prospects in 2014 and 2016 among moderates they need to have any chance of countering strong support for Democrats by minority demographics such as Hispanic Americans, African Americans and the gay-lesbian vote.
With the high political stakes come hardball tactics and strategies. The NRA has long kept a comprehensive scorecard of the voting records of legislators on gun issues, which it combines with campaign contributions to try to influence elections.
In response, a group led by New York Mayor Michael Bloomberg called Mayors Against Illegal Guns announced this week it was launching its own scorecard to identify members of Congress who vote against tougher gun laws.
"We're asked many times daily where people's elected representatives are on gun laws, and we intend to tell them, in detail," said Mark Glaze, director of the group of more than 900 mayors, of the campaign that will feature more than $1 million in cable television ads.