Who is Sen. Mark Kelly, a possible Harris VP pick?

Arizona Sen. Mark Kelly is seen as one of the front-runners to be Vice President Kamala Harris' running mate, sources have told ABC News.

Kelly hasn't immediately commented on speculation on his possibly joining the presidential ticket.

While relatively new to Capitol Hill -- having been first elected in 2020 -- the border state senator and former NASA astronaut has long been involved in national issues.

Kelly, 60, was born and raised in New Jersey with his twin brother, Scott. After graduating high school, he attended United States Merchant Marine Academy where he graduated in 1988 with highest honors.

Kelly became a U.S. Navy pilot and was deployed to Iraq during Operation Desert Storm. He flew 39 missions during the war, according to his service record. Scott Kelly was also deployed as a U.S. Navy pilot during the war, but operated on a different ship.

Kelly continued to serve as a Navy pilot following the war and in 1996 NASA selected him and his brother to be space shuttle pilots. During his 15-year NASA career, Mark Kelly logged in 54 days in space, including several missions to the International Space Station, according to his NASA records.

Kelly became closer to the world of Washington, D.C., politics after he married his second wife, Arizona Rep. Gabby Giffords in 2007, just months into her first term. Kelly was at Giffords' bedside after she was shot during an event with constituents in January 2011.

He would retire from NASA later that year as he helped his wife recuperate from her near-fatal wounds.

The couple became vocal gun control advocates and started a political action committee -- Americans for Responsible Solutions -- in 2013 which eventually evolved into the gun control non-profit Giffords. For years, Kelly would advocate for tighter gun control legislation including universal background checks, red flag laws and other measures.

Kelly, who is a gun owner, has worked with local, state and federal officials on this issue and backed candidates who shared those views.

In 2020, Kelly announced that he would get more involved in Washington and ran on the Democratic ticket for the special election that year for Arizona's U.S. Senate seat.

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Kelly won the highly watched election with just roughly 78,000 votes, beating out Republican incumbent Martha McSally. The election helped Democrats gain power in the Senate.

Kelly would be reelected two years later, beating out Republican challenger Blake Masters with over 125,000 votes.

Since he joined the Hill, Kelly has positioned himself as a moderate Democrat who has been vocal on the issues.

Kelly continues to advocate for laws and policies that enact gun control, including universal background checks.

Last month, following the U.S. Supreme Court's decision that ruled a ban on federal ban on bump stocks was illegal, the senator vowed to introduce legislation that would make them illegal nationwide.

"Banning bump stocks is common sense. They make semi-automatic guns even deadlier, and as a gun owner and someone who knows personally the damage a gun can do, I will always support efforts to protect Americans from the most dangerous weapons and devices," he said on a statement on his X page.

Following the U.S. Supreme Court's Dobbs decision overruling Roe v. Wade guaranteeing nationwide access to abortion, Kelly has been pushing for abortion access for women both in Arizona, which has a ban after 15 weeks of pregnancy, and around the country. The state legislature repealed an Arizona State Supreme Court ruling that banned all abortions this year.

"Right now, as Arizona whiplashes between two abortion bans, physicians are choosing not to come to our state and others are considering leaving, putting women's lives at risk. We must write abortion rights into law to eliminate once and for all the catastrophic consequences of Roe v. Wade being overturned," he said in a statement in May.

In the past, Kelly has criticized President Joe Biden over his policies on immigration, specifically ending Title 42, which allowed the U.S. to turn away asylum seekers during the pandemic.

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"When the president decided he was going to do something dumb on this and change the rules that would create a bigger crisis, I told him he was wrong. So I pushed back on this administration multiple times," Kelly said during a 2022 debate.

Kelly has been calling for more border agents and support in Arizona to handle the influx of migrants.

The senator commended Biden recently after the president issued an executive order in June that turned away migrants who cross illegally between ports of entry and try to claim asylum after seven consecutive days of more than 2,500 encounters.

"This doesn't change the ability for individuals to come here and seek asylum," Kelly said in an interview with PBS last month. "There will be pathways."

Kelly chastised former President Donald Trump for calling on Republicans to block a bipartisan bill that would have addressed the migrant crisis.

"The politics of the presidential election superseded everything," he told PBS. "To run away from a comprehensive piece of legislation that was going to help the Border Patrol, help CBP, [and] help communities in Southern Arizona and other states...I have never seen [it]."