The retired astronaut Mark Kelly announced on Tuesday he is running to finish John McCain’s final term in the Senate.

Kelly, 54, became a national campaigner for gun law reform after his wife, the congresswoman Gabrielle Giffords, was shot and severely injured in January 2011. He is a top Democratic recruit to take on Republican Martha McSally in what will be one of the most closely contested Senate races of the 2020 election.

McCain died in August. McSally is a former Republican congresswoman who was appointed to McCain’s seat after she lost a race for the other Arizona US Senate seat to Democrat Kyrsten Sinema in November. McSally leaned heavily on her record as the first woman to fly a combat mission as a fighter pilot, but she was hurt by her embrace of Donald Trump.

If Kelly is nominated, the race would pit a navy veteran and astronaut against a trailblazing air force pilot in the contest to replace McCain, a legendary navy flyer who was famously shot down and held captive in Vietnam.

Democrats are eagerly watching the Arizona contest, having already beaten McSally once.

Giffords was severely wounded in a mass shooting on 8 January 2011, at a meet-and-greet event outside a grocery store in Tucson. Six people were killed and 13 injured. The gunman, Jared Loughner, was sentenced to life in prison.

Kelly and Giffords have pushed Congress to enact gun control measures, with little success. They shifted their focus to state legislatures in recent years, helping strengthen background checks and domestic violence protections, among other modest reforms.

Arizona governor Doug Ducey appointed McSally to the vacant Senate seat after his first appointee, former senator Jon Kyl, resigned after a few months in office.

The seat is a top target for Democrats because McSally just lost a Senate race and Democrats posted a strong showing elsewhere in Arizona, winning three statewide contests and picking up legislative and congressional seats.

The 2020 election will decide who fills the last two years of McCain’s term. The winner will have to run again for a full six-year term in 2022.

Congressman Ruben Gallego of Phoenix is also considering a Senate run, which would likely set up a tough fight for the Democratic nomination.

Former Arizona attorney general Grant Woods, a lifelong Republican who became a Democrat and a fierce critic of Trump, announced last week that he will not run, saying he didn’t want to fight in a contested primary.