UPDATE 2-Wounded ex-U.S. Rep. Gifford's husband to run for U.S. Senate

Retired U.S. astronaut Mark Kelly, whose wife former Congresswoman Gabby Giffords was wounded in a 2011 mass shooting, plans to run for a U.S. Senate seat in Arizona, he said in a video posted to Twitter on Tuesday.

Giffords stepped down from Congress after sustaining a serious head injury in the attack but has emerged as one of the leading voices of the U.S. gun-control movement. In the 4-1/2 minute video, Kelly sits on a sofa next to Giffords, discussing his mother's history as a police officer as well as the attack on his wife.

"I thought that I had the risky job. It turned out that you were the one who had the risky job," said Kelly, a former U.S. Navy combat pilot and former space shuttle commander. Kelly will challenge Republican U.S. Senator Martha McSally, a former U.S. Air Force combat pilot who was appointed to the seat held by the late John McCain. A special election will be held in 2020 to determine who will serve the last two years of the term.

McSally lost a campaign for the state's other Senate seat to Democrat Kyrsten Sinema in November. (Reporting by Scott Malone in Boston; Editing by Nick Zieminski and Jeffrey Benkoe)