"You can't be against criminals and the dangerously mentally ill getting guns and be against background checks," he added. "That's the first line of defense."
The comments came just hours before the Senate voted to move to debate on a package of bills designed to rein in gun violence by preventing dangerous people from buying firearms — Congress's most significant examination of the nation's gun laws in almost 20 years.
Although the National Rifle Association (NRA) had endorsed universal background checks after the 1999 shooting massacre at Columbine High School, the gun lobbying group now opposes even the less stringent Manchin-Toomey proposal, which would exempt family members and neighbors from performing screenings. Along with a number of Republicans, the NRA now contends the expanded screenings would shift new burdens on gun owners and threaten Second Amendment rights.
"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 NRA said Wednesday in a brief statement.
Supporters counter that expanding background checks does nothing to limit the ownership of guns — except for those already prohibited from possessing them.
"I don't consider criminal background checks to be gun control; I think it's just common sense," Toomey said Wednesday as he unveiled his bill in the Capitol. "The worries that we hear sometimes about background checks leading to an erosion of our Second Amendment rights, it simply hasn't happened."
It's unclear if Toomey and Manchin have the 60 votes they'll need to pass their amendment through the upper chamber. While 16 Republicans voted in favor of moving to debate the proposed gun controls, many of those are expected to oppose final passage of the individual reforms, all of which will require 60 votes to get across the finish line.
Thompson, meanwhile, is readying his own background check bill, which he expects to offer later this month with Rep. Pete King (R-N.Y.).
It's unclear how many House Republicans are ready to join King in support of tougher gun laws.
Toomey, a former House member, said Wednesday that he's been in discussions with some of his former colleagues in the lower chamber, and that a "substantial number" back the notion of expanded background checks.
"There are definitely Republicans in the House that support this," Toomey said.
But House GOP leaders, historically opposed to tougher gun laws, have approached the debate very cautiously, committing only to look at what the Senate can pass, but not promising any action on it.
Thompson said he and King are talking with "about seven or eight" House Republicans in hopes of rallying their support for the bill. With polls showing that public support for expanded background checks tops 90 percent, Thompson warned GOP leaders that they ignore the issue at their own peril.
"If they for a minute think they can throttle this [and] put it on the back shelf or lock it away in the back room, they're going to be incredibly surprised by what happens across this country," Thompson said.
Behind Thompson, a group of military veterans gathered at the Capitol on Thursday to urge Congress to expand background checks surrounding gun sales. The vets highlighted a new poll, commissioned by the liberal Center for American Progress Action Fund and VoteVets.org, showing that 91 percent of veterans back universal screenings.
"I'm a supporter of the Second Amendment, but I know that that support must include keeping guns away from criminals and other dangerous individuals," said Joe Accardi, the GOP mayor of Roselle Park, N.J., and a veteran. "It's just plain crazy to think we would do anything but that."
Some liberal gun control advocates, meanwhile, are hoping there's still an appetite for universal screenings. Rep. Mike Quigley (D-Ill.) said Thursday that expanding background checks to cover all sales online and at gun shows, like the Manchin-Toomey bill would do, is "a massive improvement" over current law.
"But universal is universal," he added. "A lot of private sales are dangerous, too."