Phoenix (AFP) - From the International Space Station to Congress? Astronaut Mark Kelly is leading an unlikely political "mission" to win a second Senate seat for the Democrats in the traditionally conservative state of Arizona.

Until recently, few would have thought it possible for the Democratic Party to make major inroads in this southwestern state with a long Republican tradition.

But thanks to changing demographics and growing discontent with President Donald Trump among the state's moderate conservatives, Kelly believes he can win the upper-house seat while helping deliver Arizona's electoral college votes for Joe Biden.

Kelly, who served 25 years in the US Navy and with NASA, is drawing significant attention not just for his background and the boost he could give Biden's presidential bid, but because victory would give Arizona two Democratic senators for the first time in six decades.

"Mark Kelly has a really good shot at winning the seat," said Arizona State University politics lecturer Gina Woodall.

"This is a really big deal."

His opponent is Republican incumbent Martha McSally, also a military veteran, who was appointed by Arizona's governor last year after the death of John McCain.

McSally lost a bid for a full term in the state's other Senate seat in 2018 to Democrat Kyrsten Sinema -- the first Republican upper-house loss in Arizona since 1988.

Another loss would be truly historic, at a time when the Republicans are scrambling to retain their majority in the Senate.

"This Senate seat is one of, if not the most important race in the country... this can be the critical seat," said Woodall.

'Orbiting the Earth'

"It's all about the mission," says Kelly's campaign website, which proudly displays the candidate's previous "office" -- the ISS. A video shows Kelly spacewalking, and looking at photographs of his extraterrestrial exploits.

Kelly's campaign promises to give Arizona an "independent voice" in the Senate, with a statement on the site insisting he has "never looked at things through a partisan lens."

"Unlike the federal government, the International Space Station can't just shut down when people don't get along," he writes. 

"And when you're orbiting the Earth at 25 times the speed of sound and bad stuff starts to happen... you can't dismiss ideas based on the politics of the person offering them."

The message is intended to appeal to moderate voters in Arizona, and contrasts sharply with McSally's rhetoric, which hews closely to President Donald Trump's polarizing style and tries to paint Kelly as a radical agent of the left.

Trump repeated that message during a campaign visit to Arizona this week.

But ASU professor emeritus Richard Herrera believes McSally's close partnership with Trump could work against her.

"She did that in 2018 and lost and there's not a lot of reason to think that getting so close to him now is going to help," he told the Arizona Republic newspaper.

"The trends seem to be going in the opposite direction."

'More progressive'

On the campaign trail, Kelly has prioritized access to health care, a key Democratic issue in this election cycle.

He has also called for stricter gun controls, a cause he embraced after his wife -- former congresswoman Gabby Giffords -- survived a gunshot wound to the head during a 2011 assassination attempt.

Still, Kelly stops short of calling for an outright ban, and is himself a gun owner.

"He's pro-Second Amendment... at the same time, he has more progressive views on other things," said Woodall.

Boosting the Democrats' chances of courting moderate voters in Arizona is the state's other senator, Sinema, who is "one of the few bipartisan heroes left that we have" in the upper house, Woodall adds.

Her popularity could serve Biden well -- if he wins Arizona, he would be the first Democratic presidential candidate to do so since Bill Clinton in 1996.

McCain's widow Cindy is also backing Biden and working with his campaign.

"The Kelly campaign is bringing out a lot of Democratic voters, and we've had a huge increase in the number of Arizonans registering as Democrats in recent months," University of Arizona associate professor of politics Samara Klar.

"There's going to be a lot of people who come out to support Kelly and end up voting for Biden as well."