Mark Kelly, husband of former US Rep. Gabrielle Giffords, says the gun-control organization the couple has founded will work to defeat Sen. Jeff Flake (R) in the next election if the Arizona senator votes against expanding background checks to include more gun buyers.

Mr. Kelly, a retired Navy captain and astronaut, said Senator Flake is “a good friend” of Ms. Giffords. But he gave this warning Tuesday at a Monitor-hosted breakfast with reporters: “If there was the right candidate out there and he [Flake] didn’t support this legislation,… you know friendship is one thing … saving people’s lives, especially first-graders', is another.”

On Monday, Flake posted on Facebook a statement that he will oppose the bipartisan legislation on background checks drafted by Republican Sen. Pat Toomey of Pennsylvania and Democratic Sen. Joe Manchin of West Virginia. The measure “simply goes too far,” Flake said.

Kelly criticized Flake's action.

“Senator Flake’s timing for his posting to his Facebook was not ideal in my opinion,” he said. On Tuesday afternoon, Flake is slated to join Giffords and Kelly, along with Vice President Joseph Biden and House Speaker John Boehner, to dedicate a room in the House in honor of Gabriel Zimmerman. He was a member of Giffords's staff and was killed in the attack that gravely wounded her in January 2011.

The warning to Flake from the cofounder of Americans for Responsible Solutions comes amid reports that prospects are dimming for new gun-control legislation. Republican opposition to expanded background checks is growing in the Senate, “enough to put the proposal’s fate in jeopardy,” The Associated Press reported Tuesday. Likewise, a Politico story on Tuesday said gun control is “struggling in the Senate and facing outright rejection in the House.”

Kelly, whose comments can be seen on C-SPAN, said he is “optimistic that we can get something passed.” But he conceded that “we do have a problem with a lot of United States senators with regards to this bill.” He added: “There are a lot of US senators that are just looking for a reason to get to no. And I experienced this personally last night when I was shown the Facebook posting of Senator Jeff Flake of Arizona, Gabby’s good friend.”

The Arizona Republic reported that Flake spent considerable time at the hospital so he could help when Giffords was wounded and Kelly thanked him in the book he co-wrote with Giffords. As The Huffington Post notes, Flake sat with Giffords during her appearance at the 2013 State of the Union message in February.

Adopting a carrot-and-stick approach, Kelly said that he sees Flake “as a reasonable person and that when I explain to him in person why his concerns with this bill are unfounded, I think we can get him to come around.”

Before the shooting, Kelly and Giffords alternated between his home in Texas and hers in Tucson, Ariz. But Kelly said he has recently sold his home in Texas and is registered to vote in Arizona. Kelly said he had not “given any thought” to running for office himself. “Gabby and I want to help in solving this problem and that is our No. 1 priority,” he said.