A Democrat backs Republicans on gun control? It just happened.

To some Democrats, it looks like a political stab in the back.

This week, a leading gun-control group that was founded by a former Democratic member of Congress endorsed two Republican incumbent senators. If that weren't bad enough, the endorsements came in toss-up races that Democrats would dearly love to win.

It was a highly unusual crossover on a polarized issue – and in an election year in which control of the Senate is up for grabs. It was also a sign of political pragmatism when it comes to gun regulation, say political observers.

“The only way this gets resolved is if someone gets in the middle,” says pollster G. Terry Madonna, of Franklin & Marshall College in Lancaster, Pa. Given that most Democrats already support broader background checks and other gun-control measures, that someone is Republicans, says Mr. Madonna.

Indeed, that factored into this week’s endorsement of Republican Sens. Pat Toomey of Pennsylvania and Mark Kirk of Illinois by Americans for Responsible Solutions, a political action committee, or PAC. The group was founded by former Democratic Rep. Gabrielle Giffords of Arizona and her husband, astronaut Mark Kelly – both gun owners – founded the PAC after the December 2012 shooting at Sandy Hook Elementary School in Newtown, Conn. Ms. Giffords herself was severely injured in a 2011 mass shooting that killed six people.

In an opinion piece published by CNN on Monday, the couple admitted that the endorsements were a “difficult” decision. They praised the Democratic challengers in both Senate races and said they would be consistent votes for gun-safety regulation in Congress.

However, the PAC’s founders said it was important to stand by friends who had championed smart gun laws in Congress – including Republicans. After the 2012 mass shooting at Sandy Hook, Senators Toomey and Kirk “broke from the gun lobby” and backed a bill, co-sponsored by Toomey and Joe Manchin (D) of West Virginia, to extend background checks to gun shows and Internet sales.

The bill has yet to pass, despite overwhelming support for the idea from Americans. It is opposed by the National Rifle Association and many Republicans in Congress.

“Our nation needs more Republican elected officials to stand with the vocal majority of Americans who support common sense steps that help keep guns out of the wrong hands and prevent gun tragedies,” said Peter Ambler, executive director of the PAC, according to a statement from the Toomey campaign.

This is the second endorsement of Toomey by a high-profile gun-control group this month. Earlier, former New York City Mayor Michael Bloomberg’s PAC, Independence USA, also backed the Pennsylvania Republican. Also, the daughter of the Sandy Hook principal who was killed in the Newtown, Conn., shooting appeared in an ad for Mr. Bloomberg’s PAC, praising Toomey for reaching across the aisle to support gun-control legislation.

Democrats shoot back

Democrats in Pennsylvania and Illinois have shot back. In Pennsylvania, Toomey trails challenger Katie McGinty by only 3 points, according to a Quinnipiac poll from earlier this month. The McGinty campaign on Wednesday touted the endorsement of another gun control group, CeaseFire Pennyslvania, and the campaign as well as state Democrats point to holes in Toomey's record on guns.

“Pat Toomey isn't a moderate on guns, period,” said Pennsylvania Democratic Party spokesman Preston Maddock in a statement. “He's stood in the way of common sense gun safety laws and he would happily keep Mitch McConnell as the Senate Majority Leader, meaning that gun safety laws have almost no chance in the Senate.”

A similar point was made by the campaign of Rep. Tammy Duckworth (D) of Illinois, a US military veteran who is running against Senator Kirk in a race that is also considered a toss-up by the independent Cook Political Report.

“It’s a campaign year. Democrats are obviously not happy about what was done,” says Jennifer Duffy, who tracks Senate races for the Cook Political Report.

How the endorsements could help

Kirk and Toomey can use their endorsements to shore up their moderate credentials with voters in big cities and their suburbs, political observers say. Chicago has suffered mightily from gun violence and swing voters in the Philadelphia area are a must for Toomey if he wants to escape the downdraft of Donald Trump.

But neither pollster Madonna nor Ms. Duffy believe the gun issue will be a deciding issue in these races.

What’s notable, says Duffy, is that a gun-control group is backing Republicans, a recognition that if there is ever to be an agreement in Congress, “it’s going to take some Republicans." Gun-control supporters, she says, “have to reach out.”