Key U.S. senators on Wednesday unveiled a bipartisan deal to expand background checks for buyers seeking to purchase guns, as the upper chamber is poised to vote on the biggest change in gun laws in twenty years.

Senators Joe Manchin and Pat Toomey unveiled the deal in Capitol Hill, saying the Sandy Hook Elementary School shooting late last year had changed the debate on guns, convincing them new gun control measures were warranted.

"Nobody here in this great Capitol of ours with a good conscience could sit by and not try to prevent a day like that from happening again," said Manchin, a Democrat. "I think that's what we're doing."

The proposal would require background checks for sales at gun shows and online, but exempts personal transfers from such checks. It also calls for the creation of a "commission on mass violence" to study mass shootings, and find ways to prevent them.

"I have to tell you candidly that I don't consider criminal background checks to be gun control," said Toomey, a Republican. " I think it's just common sense."

U.S. commentators said the Manchin-Toomey measure would be used as a political cover for Republicans to vote Thursday and exceed the 60-vote requirement to allow the Senate to end a filibuster and proceed to floor debate on gun control legislation.

However, how many Republicans would support the measure is unclear, the National Rifle Association has already voiced its opposition to the proposal.

Toomey said another Republican, Senator Mark Kirk, will back the measure, but would not predict whether other Republicans would come onboard.