Ellison's support came just a few days after Sanders was endorsed by Rep. Raúl Grijalva (D-Ariz.) during a rally Friday in Tucson, Ariz., that drew an estimated 13,000 people.
Ellison and Grijalva are both co-chairmen of the Congressional Progressive Caucus, which Sanders helped co-found as a House member in 1991.
While their backing is a boost to Sanders, it also serves as reminder of a big advantage that Hillary Rodham Clinton maintains in the Democratic race even as Sanders has gained or surpassed her in early state polling: More than 100 members of Congress have already endorsed Clinton.
In the nomination process, such party elites are more than just cheerleaders. Roughly one-fifth of the delegates who will pick the Democratic nominee are super delegates — elected officials and other party leaders who are not bound by voting in their states. And to this point, they’ve broken overwhelmingly in Clinton’s direction.
In explaining his support, Ellison said that Sanders has "the ability to create a renaissance in voter participation, which was at its lowest in decades this past election cycle."
We’ve all seen the massive crowds he is attracting, and I think that is a testament to his message connecting with people — people we will need to turn out in November," Ellison said.
One of Sanders's first eye-popping crowds was at an event in Ellison's home state that drew more than 3,000 people to a venue in Minneapolis, according to the Minneapolis Star-Tribune.
“Keith Ellison is one of the great progressive leaders in the country leading the fight for the rights of working families and the environment,” Sanders said in a statement Monday. “I look forward to working with him to create a government which represents all Americans and not just the billionaires.”