They are perfect for this role. She is a courageous survivor of a gun attack, a former member of Congress from a western state, a longtime hunter and a supporter of gun rights. He is a combat veteran, navy pilot and space shuttle commander. The National Rifle Association can’t paint them as effete foes of the Second Amendment.

In January, Ms. Giffords delivered emotional testimony on the measure to the Senate Judiciary Committee. She and Mr. Kelly personally lobbied members. Before the Senate vote on the proposal last month, she sought out Jeff Flake, a Republican senator from Arizona and a friend from her House days, and blurted out, “Need,” as in we need you. Unlike his Arizona colleague John McCain, who backed the background checks compromise, Mr. Flake voted no. The measure failed; since then, polls show a drop in Mr. Flake’s popularity in his home state.

The coalition anchored by Mr. Kelly and Ms. Giffords, and working with the White House, is determined to reverse the Senate defeat on background checks. Supporters of the measure had a majority of the chamber, but the Republican leadership encouraged a filibuster and the measure fell 5 votes shy of the 60 needed to proceed.

Ms. Giffords and Mr. Kelly then bought $350,0000 of radio ads in five states, attacking opponents of the measure, including the Senate Republican leader, Mitch McConnell. They also praised backers who are likely to face attacks from the National Rifle Association next year, like Mary L. Landrieu of Louisiana and Kay R. Hagan of North Carolina.

An ad they produced in support of Ms. Hagan had the perfect narrator: a North Carolina sheriff nicknamed Moose.

The toughest broadside was directed at Senator Kelly A. Ayotte, a New Hampshire Republican who was the only New Englander to vote against the measure. “Remember that ad Kelly Ayotte ran saying she’s one of us?” the Responsible Solutions commercial asks. It goes on to say that Ms. Ayotte “went to Washington” and ignored the feelings of New Hampshirites about background checks. The freshman Republican lawmaker, Granite State politicians say, is squirming over this ad and the criticism she encountered at a recent town meeting.

With a few cosmetic changes to the proposal, background check strategists are counting on pressuring Ms. Ayotte, Mr. Flake and several other Republicans, as well as a few Democrats, to switch their votes and help pass the measure. It then would face an uphill battle in the Republican-controlled House; no one could more effectively bring heat on her former colleagues than Ms. Giffords.