“I always knew that I was going to serve this country in some way,” Kelly, a child of two former police officers, said in a video posted to his Twitter account.

A child of two former police officers, Kelly is an engineer by training as well as a retired NASA astronaut and U.S. Navy captain. He flew 39 combat missions in Operation Desert Storm from 1990 to 1991.

Kelly, running as a Democrat, would face Republican Martha McSally, who was the first woman to fly a jet fighter in combat, should he win his primary. McSally ran in 2018 for Arizona’s other Senate seat, but lost to Democrat Kyrsten Sinema, who previously served in the House of Represenatives.

Arizona Governor Doug Ducey initially named Jon Kyl, a former senator, to replace McCain. But Kyl had only committed to serving until 2019. Ducey then named McSally to take McCain’s place until a special election in 2020.

The 54-year-old Kelly is the husband of former congresswoman Gabrielle Giffords, who survived an assassination attempt in 2011. Kelly has co-authored two books with Giffords and written two addition books for children. He became active in gun control efforts after the attack on Giffords.

“What I learned from my wife is how you use policy to improve people’s lives,” Kelly said in the announcement video.