Two key senators have reached a deal to expand background checks to firearms sales at gun shows and on the Internet, sources close to the negotiations said early Wednesday.
Sen. Pat Toomey, a conservative Pennsylvania Republican, plan to announce the deal Wednesday with West Virginia Democratic Sen. Joe Manchin, who holds an A rating from the National Rifle Association.
The two have been working on a compromise proposal that could draw Republican support for expanding background checks. On Tuesday evening, the two had an agreement in principle, and spent the night hammering out the final details.
The compromise doesn't go as far as the universal background checks that President Barack Obama first envisioned in the wake of the Newtown shootings. The Manchin-Toomey compromise will include exemptions for some transactions, such as those between family members.
Outlines of the compromise have been circulated to the National Rifle Association, and sources close to the negotiations said it's unclear where the group stands although the powerful lobby is unlikely to support it.
Senate Democrats, meanwhile, set up a possible Thursday vote on gun legislation.
The deal between Toomey and Manchin represents a major breakthrough for a package of new gun laws that Obama proposed in the wake of the shooting at Sandy Hook Elementary School on Dec. 14.
Support from the conservative Toomey, who also carries an A rating from the NRA, could give other, more moderate Republicans cover to vote in favor of a bill to expand background checks for gun sales beyond just those conducted through licensed dealers.
In recent days, Obama's gun control agenda has been imperiled on Capitol Hill. While Democratic leaders have promised votes on an assault weapons ban and new limits on high capacity magazines, neither can realistically pass the Senate. And a deal on background checks has eluded Democrats for months -- threatening to leave the president with only stricter gun trafficking laws to show for a prolonged, emotional national plea for tighter restrictions on firearms after 20 young children and 6 educators were gunned down in Connecticut.
But there was new momentum for gun legislation Tuesday as Republican senator after Republican senator announced they wouldn't support a filibuster that would prevent gun legislation from even coming up for debate. A trio of conservatives -- Sens. Ted Cruz of Texas, Rand Paul of Kentucky and Mike Lee of Utah -- are leading the filibuster effort, with support from Senate Minority Leader Mitch McConnell. But as Tuesday wore on, as many as 10 Republican senators said they could not support it or left the door open to allowing Democrats to bring the measure up on the floor.
"The purpose of the United States Senate is to debate and to vote and to let the people know where we stand," Sen. John McCain, R-Ariz., said on CBS' Face the Nation on Sunday. "What are we afraid of?"
On Tuesday night, Reid officially filed gun legislation that's been written by Democrats. It sets up a possible Thursday vote to open debate on guns. Senate aides said debate on gun legislation could continue through next week and even into the following week. The Manchin-Toomey compromise would likely be the first amendment offered to the package.
The vote to open debate is tricky for some Democrats who hail from conservative states like Arkansas, where the NRA and other pro-gun groups hold significant sway. But Republican movement in favor of it could help protect them and increases the chances that the vote will succeed.
Now, a key question is how conservatives who've signed on to filibuster the gun bill decide to proceed. They haven't ruled out taking a stand on the Senate floor, similar to Paul's 14-plus-hour talkathon opposing drone strikes on American citizens.
That has some Republicans on edge. One member of Senate leadership, speaking on condition of anonymity so as not to violate personal confidences, said there's a sense among the top GOP lawmakers that such a public display could further damage the already-battered Republican brand.
But McConnell, who's up for re-election in 2014, vowed Tuesday to stay the course and filibuster the bill.
"It clearly had no bipartisan support in committee," he said.
NBC News' Frank Thorp and Mike O'Brien contributed to this report.
This article originally appeared on NBCNews.com.