HARRISBURG — Gun-control politics put down more roots in Pennsylvania’s race for U.S. Senate on Monday as former Democratic congresswoman Gabby Giffords endorsed the re-election bid of Republican incumbent Pat Toomey over Democratic challenger Katie McGinty.

Giffords, who was gravely wounded in a 2011 mass shooting in Arizona, and her husband, Mark Kelly, made the endorsement in an editorial on behalf of their organization, Americans for Responsible Solutions.

Toomey, they said, “broke from the gun lobby” in a 2013 vote to expand background checks to gun purchases online and at gun shows. The bill — a response to the shooting rampage at Connecticut’s Sandy Hook Elementary School in 2012 — ultimately failed amid Republican opposition.

Gun control is already the subject of two TV ads running in Pennsylvania’s U.S. Senate race, which is viewed as crucial to determining control of the chamber next year. Polls show a neck-and-neck race between Toomey and McGinty.

McGinty’s campaign said that her positions on gun control are more in line with Giffords’ and that Toomey has hardly strayed from the National Rifle Association’s positions in a flurry of recent votes.

On Monday at the Pennsylvania Press Club in Harrisburg, McGinty attacked Toomey as doing little more than lending his name to the background-check legislation before abandoning it. He touts his “A rating” with the NRA and twice voted against measures to prevent terrorism suspects from buying guns, McGinty said.

“He’s not been a fighter for common-sense gun safety,” McGinty said.

Democrats also noted that McGinty supports a ban on the sale of assault weapons and limits on magazine capacity. Toomey does not.