A gun-control group launched by former Rep. Gabby Giffords (D-Ariz.) is throwing its support behind Sens. Pat Toomey (Pa.) and Mark Kirk Mark Steven KirkLiberal veterans group urges Biden to name Duckworth VP On the Trail: Senate GOP hopefuls tie themselves to Trump Biden campaign releases video to explain 'what really happened in Ukraine' MORE (Ill.), two Republican senators facing tough reelection bids.

"[They] broke from the gun lobby and supported a bill to help prevent felons, domestic abusers and the dangerously mentally ill from obtaining firearms at gun shows and online" after the 2012 shooting at Sandy Hook Elementary School in Newtown, Conn., Giffords and her husband, Mark Kelly, who launched Americans for Responsible Solutions PAC, wrote in a CNN op-ed Monday.

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Toomey and Sen. Joe Manchin Joseph (Joe) ManchinNames to watch as Trump picks Ginsburg replacement on Supreme Court Momentum growing among Republicans for Supreme Court vote before Election Day Gardner on court vacancy: Country needs to mourn Ginsburg 'before the politics begin' MORE (D-W.Va.) introduced a gun-control proposal in the wake of the 2012 shooting aimed at strengthening background checks. The measure, which was supported by Kirk, failed to overcome a procedural hurdle in the Senate.

Giffords and Kelly started Americans for Responsible Solutions PAC after the former Democratic congresswoman was shot in 2011 in Arizona.

Toomey — who has put public safety and law enforcement at the center of his reelection bid — said Monday that he would introduce a new gun-control bill next year if he wins in November.

The group's endorsement comes less than a week after Erica Smegielski, the daughter of the Sandy Hook Elementary School principal who was among the 26 killed at the Connecticut school, backed Toomey. Former New York City Mayor Michael Bloomberg's Independence USA PAC also endorsed the Pennsylvania Republican this month.

Pennsylvania Democrats quickly hit back at the endorsement, saying Toomey "isn't a moderate on guns, period."

"He's stood in the way of common sense gun safety laws and he would happily keep Mitch McConnell Addison (Mitch) Mitchell McConnellOcasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat McConnell locks down key GOP votes in Supreme Court fight Video shows NYC subway station renamed after Ruth Bader Ginsburg MORE [R-Ky.] as the Senate Majority Leader, meaning that gun safety laws have almost no chance in the Senate," said Preston Maddock, a spokesman for the state party.

Toomey and Kirk are two of a handful of GOP senators running for reelection in purple states previously carried by President Obama. With Republicans defending 24 seats, Democrats need to net five — or four if they also retain the White House — to win back the Senate majority.

Democratic Senate candidate Katie McGinty is currently leading Toomey in the polls by less than 3 points, according to RealClearPolitics.

— This story was updated at 11:16 a.m.