Retired astronaut Mark Kelly, who became a prominent gun-control advocate after his wife, former US Congresswoman Gabrielle Giffords, was shot in a failed assassination attempt, has announced he will run to finish John McCain’s last term in the US Senate.

If he wins the Democratic nomination, Mr Kelly would take on Republican Martha McSally in what is expected to be one of the most closely contested Senate races of the 2020 election.

Mr Kelly described himself as an independent-minded centrist who will take a scientist’s data-driven approach to solving problems such as climate change, wage stagnation and health care affordability.

“You see a lot of partisanship in Washington and a lot of polarisation, and to some extent we’ve created that,” Mr Kelly said.

“It’s going to take people who are more independent to fix it.

“Arizonans value independence.”

If Mr Kelly is nominated the race would pit the Navy veteran and astronaut against Ms McSally, a trailblazing Air Force pilot, in the contest to replace Mr McCain, a legendary Navy flyer who was famously shot down and held captive in North Vietnam.

John McCain visits the UK
The late John McCain (John Stillwell/PA)

Ms McSally is a former Republican congresswoman who was appointed to Mr McCain’s seat after she narrowly lost to Democrat Kyrsten Sinema last November in the race for outgoing Republican Jeff Flake’s seat.

Ms McSally leaned heavily on her record as the first woman to fly a combat mission, but she was hurt by her embrace of President Donald Trump.

The 2020 election will decide who finishes the last two years of Mr McCain’s term.

The winner would have to run again for a full six-year term in 2022.

Democrats are eagerly watching the Arizona contest, having already defeated Ms McSally.

The party is also gauging whether Arizona could be competitive at the presidential level in 2020.

Mr Trump won the state in 2016.

Mr Kelly has never held elected office.

He flew combat missions during the first Gulf War before becoming an astronaut along with his twin brother, Scott Kelly.

He flew four space missions over 10 years and commanded the Space Shuttle Endeavour in 2011.

Mr Kelly and Ms Giffords have pushed Congress to enact gun control measures with little success.

They shifted their focus to state legislatures in recent years, helping to strengthen background checks and domestic violence protections.

Election 2020 Senate Arizona
Former Representative Gabrielle Giffords, listens as her husband Mark Kelly, left, speaks on Capitol Hill in Washington (Susan Walsh/AP)

Ms Giffords was severely wounded in a mass shooting on January 8 2011.

The shooting at a meet-and-greet event in Tucson left six dead and 13 injured.

Ms Giffords played a prominent role in the four-minute video launching Mr Kelly’s campaign.

“I thought then that I had the risky job,” Mr Kelly said to Ms Giffords.

“Turned out, you were the one that had the risky job.”

Mr Kelly said Ms Giffords, who has been a rising Democratic star before the shooting, will join him frequently during campaign appearances.

Mr McCain, a legendary and beloved Arizona politician, died last year from an aggressive form of brain cancer after more than three decades in the Senate.