Mark Kelly, the husband of former Rep. Gabrielle Giffords (D-Ariz.), is threatening to back an election challenge against Sen. Jeff FlakeJeffrey (Jeff) Lane FlakeSpokesman: Flake’s vote on tax reform will have nothing to do with Trump Trump slams Flake over hot-mic comments: Senator's career is 'toast' Bannon: McConnell 'picking up his game' because of our 'insurgent movement' MORE if the Arizona Republican votes against legislation to expand background checks.

At a Christian Science Monitor breakfast on Tuesday, Kelly said he would back an opponent to Flake if the "right candidate" was available and if the senator failed to support a background check bill, according to reports.

Flake, who is not up for reelection until 2018, has been close to Kelly and Giffords. But Kelly said the issue trumped their ties.

"You know, friendship is one thing," Kelly said. "Saving people's lives, especially first-graders, is another thing."

Kelly's comments came after Flake said he planned to vote against background check legislation in a statement on his Facebook page.

"A lot of senators are looking for a reason just to get to 'no,' " Kelly added. "I experienced this personally last night when shown the Facebook posting of Sen. Jeff Flake, Gabby's good friend. He posted that he intends to vote ‘no’ on this legislation."

Kelly also said that if lawmakers "won’t stand up and do what the American people are asking, we will be there to replace them," according to reports.

Giffords was shot in the head during a constituent meeting in Tucson, Ariz., in early 2011. 

Since leaving office, Giffords and her husband have become strong advocates for tougher gun control legislation. They founded Americans for Responsible Solutions, a political action committee they say will back pro-gun-control lawmakers and counter the muscle of the National Rifle Association.

Giffords and Kelly plan to raise $20 million in time for the 2014 midterms, but have not yet set any lawmakers in their sights.

Americans for Responsible Gun Solutions is also looking to collaborate with the PAC funded by New York City Mayor Michael Bloomberg, Independence USA, which played a pivotal role in helping Rep. Robin Kelly (D-Ill.) win a recent House special election. 

Kelly suggested that Flake may not yet have “read the bill” drafted by Sens. Joe ManchinJoseph (Joe) ManchinDemocrats scramble to contain Franken fallout  Overnight Finance: House passes sweeping tax bill in huge victory for GOP | Senate confirms banking regulator | Mulvaney eyed for interim head of consumer agency Overnight Regulation: Senators unveil bipartisan gun background check bill | FCC rolls back media regs | Family leave credit added to tax bill | Senate confirms banking watchdog MORE (D-W.Va.) and Pat Toomey (R-Pa.), which would expand background checks to all gun show and Internet sales.

Kelly said he would talk to Flake personally later, and hoped he could change the senator's mind.

"I think we can get him to come around," Kelly said.