Two prominent senators announced a proposed compromise on the thorny issue of gun background checks, urging mandatory checks for gun-show and Internet sales while exempting certain private transactions -- ahead of a key test vote Thursday.

The proposal Wednesday from Sen. Joe Manchin, D-W.Va., and Sen. Pat Toomey, R-Pa., who had been meeting for days on the issue, was pitched as a bipartisan compromise and one aimed at easing opposition to firearms legislation as it heads to the Senate floor.

"This amendment is a genuine compromise," Toomey said.

"This is common sense. This is gun sense," Manchin added.

The proposal, which would be voted on as an amendment, would expand background checks for sales at gun shows and online but exempt some other transactions like personal sales among hunters and family members.

"Personal transfers are not touched whatsoever," Manchin said. The senators also called for a national commission on mass violence to examine the "culture of violence" in the country.

Currently, background checks are required only for sales handled through licensed gun dealers.

The National Rifle Association quickly put out a statement criticizing the proposal.

"Expanding background checks at gun shows will not prevent the next shooting, will not solve violent crime and will not keep our kids safe in schools," the group said. "We need a serious and meaningful solution that addresses crime in cities like Chicago, addresses mental health deficiencies, while at the same time protecting the rights of those of us who are not a danger to anyone."

It's unclear whether the proposal, by two lawmakers who are on the conservative end of their respective parties, will sway wavering senators ahead of a Thursday test vote. Senate Democratic Leader Harry Reid, in setting up the vote, acknowledged Tuesday he didn't yet know if it has enough support.

With more than a dozen Republicans threatening to filibuster, they could require a 60-vote threshold -- meaning Reid needs Republicans to cross over.

Manchin, after briefing Democrats Tuesday, said the rights of law-abiding gun owners will be protected.

Manchin and Toomey said the important thing is to make sure the background check system applies to those with records of criminal behavior and mental illness -- by expanding that system beyond just licensed dealers.

The White House has pointed to public polling showing broad support for a near-universal background check system. But some conservatives and other gun-rights advocates worried that the new system could be a burden to law-abiding gun owners, particularly when it comes to casual transactions among hunters and family members.

Sen. Chuck Grassley, R-Iowa, warned Tuesday that the bill would be a "step toward gun registration" by requiring recordkeeping for private sales -- though Toomey said Wednesday that the new proposal does not require recordkeeping.

With a vote set up for Thursday, Grassley also complained that senators were "being asked to take a leap into the unknown" -- noting that, with the Manchin-Toomey proposal, the language on background checks could change in a matter of days.

Reid was proceeding to a vote as families of the victims of the Newtown school shooting -- which prompted calls for new gun control measures -- visited Capitol Hill offices this week to lobby for the legislation.

The gun legislation Reid wants the Senate to debate would extend the background check requirement to nearly all gun sales. Assuming the deal between Manchin and Toomey is completed, Reid would try to replace that language with their agreement once debate begins, a move that would require a vote.

The overall gun bill also tightens federal laws against illegal gun sales and slightly increases federal aid for school safety. A proposal to renew and expand the assault-weapons ban, along with a ban on high-capacity magazines, has been dropped from the main bill though it will likely get a vote as an amendment.

Some moderate Democrats were remaining noncommittal and could oppose opening the gun debate. They include Sens. Mark Begich of Alaska and Mark Pryor of Arkansas, who are seeking re-election next year. There are 53 Senate Democrats and two independents who lean Democratic.

The National Rifle Association opposes Obama's effort and is urging its members -- it claims nearly 5 million -- to tell lawmakers of their opposition.

The Associated Press contributed to this report.