In the months leading up to the first presidential primaries, Sanders consolidated the support of many progressive elected officials and groups. But Justice Democrats, an organization founded by former Sanders aides, was an exception. Instead of backing Sanders or the other progressive standard-bearer in the race, Massachusetts Sen. Elizabeth Warren, it said positive things about them both.

Justice Democrats' endorsement comes days after Warren dropped out of the race. She has not yet backed another presidential candidate, and it is possible she could decide not to issue an endorsement. But Warren is facing increasing pressure from Sanders' grassroots supporters to form a united progressive front and back him.

While Justice Democrats had not previously endorsed a candidate, an affiliated 501(c)4 has funded digital ads criticizing Biden and Pete Buttigieg's records.

“In daring us to connect with the pain of the diverse working class of this country and to fight for solutions that match the scale of the problems we face, Bernie and millions of his grassroots supporters from across the country have transformed the Democratic Party and our nation," Rojas added in a statement.