PHOENIX (AP) — Arizona Senate candidate Mark Kelly said Monday he'll vote for Joe Biden in the Democratic presidential primary, throwing his support behind the former vice president as Biden looks to solidify moderate Democrats against Bernie Sanders.

Kelly is a retired astronaut who has positioned himself as a centrist in one of the most hotly contested 2020 Senate races. His endorsement ends months of silence on his thinking in the presidential contest and adds to a pile-on of moderate supporters for Biden a day ahead of the Super Tuesday primaries.

“Joe Biden understands the challenges Arizonans face and knows what it’s like to be knocked down, get back up, and keep serving others,” Kelly wrote on Twitter. “We need a president who will unite us and find common ground to get things done. That’s why I’ll be voting for Joe Biden.”

Biden also picked up support Monday from Sen. Amy Klobuchar and Pete Buttigieg, who ended their own presidential campaigns amid deepening concern among the Democratic establishment about Sanders, the Vermont senator and self-described democratic socialist whose success in early states has propelled his campaign.

Kelly has moved to distance himself from Sanders, telling reporters last month, “I'm a capitalist.” But he's pledged to support the Democratic nominee for president, even if it's Sanders.

Long a Republican stronghold, Arizona has become friendlier to Democrats over the past four years. President Donald Trump won the state by just 3.5 points in 2016, and Democrat Kyrsten Sinema in 2018 became the first Democrat here to win a U.S. Senate seat in three decades.

Many Democrats worry that Sanders' self-identity as a democratic socialist and his far-left politics won't play well with the higher-income suburbanites who would be key to Kelly's chances in Arizona. Kelly is married to Gabrielle Giffords, who survived a shooting when she was a congresswoman.

Republican Sen. Martha McSally, the incumbent who was appointed to finish the term of the late Sen. John McCain, has enthusiastically backed Trump and appeared with the president at a rally in Phoenix last month.