The move provides further evidence of Biden's growing momentum, which is now expanding outside the Democratic Party's moderate circles.

"With the Arizona Primary less than two weeks away, I'm proud to endorse Joe Biden for President. As a combat veteran I can think of no better leader to serve as our Commander in Chief," Gallego said in a statement. "I know Joe Biden has the wisdom, experience, compassion, and empathy to beat Donald Trump and get our government working again."

Gallego had initially backed California Sen. Kamala Harris, but remained uncommitted since she left the race in December.

Last year, progressives pushed Gallego to run in the Arizona's Democratic Senate primary against centrist former astronaut Mark Kelly . The draft campaign was led by Latino Victory and Democracy for America and backed by fellow Arizona Rep. Raúl Grijalva, a former Congressional Progressive Caucus co-chair who endorsed Massachusetts Sen. Elizabeth Warren in the 2020 presidential primary.

Latino Victory's former president, Cristobal Alex, is now a senior adviser to the Biden campaign.

The progressive group Democracy Fforor America endorsed Sanders on Monday after an overwhelming number of members voted to support the senator. The group said in a statement that Sanders was the candidate who was "energizing our base, connecting with those who have long felt ignored by our politics, and electrifying the next generation of Americans we need to beat Donald Trump."

Sanders is banking on support from the Latino community to carry him to victory in Arizona next week in the same way it helped him win Nevada by a large margin. Sanders devoted an enormous effort to organizing among Latinos, and it paid off. Entrance polls in Nevada showed Sanders winning a stunning 53% of Latino voters, nearly three times as much as the next closest competitor (Biden at 17%).

Gallego, an Iraq War veteran, ultimately decided not to seek the seat currently held by Republican Sen. Martha McSally and endorsed Harris in the presidential primary last fall. He backed Hillary Clinton over Sanders in 2016.

Both Gallego and Grijalva, who was Sanders' first congressional endorser during his 2016 run, attended a 2018 rally with Sanders in Phoenix. Neither ended up supporting his 2020 presidential campaign.

Buttigieg, Klobuchar and O'Rourke endorsed Biden on Monday, right before the Super Tuesday contests. CNN projected Biden would win 10 out of 14 states on Super Tuesday, including a dramatic upset victory in Texas and surprise wins in Minnesota and Massachusetts. Sanders was projected to win three states: Colorado, Vermont and Utah. California has not been called as of Friday evening.

This story has been updated with additional information.