Giffords, the Arizona Democrat who was shot in the head during an appearance in front of a Tucson supermarket in 2011, called Clinton a "tough" and "courageous" candidate and pledged her support because Clinton "will stand up to the gun lobby."
"Speaking is hard for me," Giffords said, a nod to the fact that she is still recovering from the near-fatal shooting. "But in January 2017, I want to see these words, 'Madame President.'"
Clinton, clearly moved by Giffords' words, pledged to take on gun control as president.
"I have heard the stories, I have seen the news reports about the ones who were killed," Clinton said. "What is wrong with us? How can we continue to ignore the toll this is taking on our children and our country?"
After outlining her pledge to take on guns, Clinton said the "facts cry out for action" and put pressure on voters to turn her call "into a voting issue."
"None of it will stick if it is not a voting issue," Clinton said. "And as you go to caucus Monday night, please think about this."
She added,"You know, the response whenever we make this case is 'Well, you can't stop everybody.' Well, I am not expecting we can stop everybody, but I think stopping some or a lot is a pretty big deal. Saving lives is important to all of us."
Mark Kelly, Giffords' husband and an astronaut, also spoke at the event, telling the audience that his family has "come to realize that there is only one candidate that is going to build on the progress that has been made over these past eight years and not roll it back, and there's only one candidate that's willing to take on the tough fights. And that candidate is Hillary Clinton."
Kelly and Giffords, who endorsed Clinton earlier this month
, have not always been aligned with Clinton. Kelly endorsed Sen. John McCain in 2000 and Giffords backed then-Sen. Barack Obama in 2008.
After the 2011 shooting, though, the couple turned their focus to Giffords' rehabilitation and pushing for more gun control.
Clinton has been outspoken about gun control during her run for president, including the issue in nearly every speech she delivers.
In the wake of last year's church shooting in Charleston the killed nine people, Clinton said it was time to "tackle" the issue of guns "with urgency and conviction."
"The stakes are too high, the costs are too dear, and I am not and will not be afraid to keep fighting for common sense reforms and along with you, achieve those on behalf of all who have been lost because of this senseless gun violence in this country," she said in June.