Calling him a "reliable progressive partner" -- one of four times he used the word "progressive" in his email to supporters -- O'Malley says Van Hollen has been a leader in the state on issues such as the environment, gun control and education.
"In my time serving on the Baltimore City Council, as mayor of Baltimore, and as governor, Chris was a reliable, progressive partner in the General Assembly and in Congress," O'Malley wrote. "Chris knows what’s needed to move Maryland forward, and he knows how to get things done."
The endorsement is the second O'Malley has made in a Senate primary this cycle. He backed Braddock Mayor John Fetterman over former congressman Joe Sestak and Katie McGinty in Pennsylvania's competitive Democratic primary.
O'Malley, who ran a spirited campaign for president but who dropped out after a lackluster finish in the Iowa caucus, has notably not endorsed a candidate for the presidential nomination despite a long relationship with front runner Hillary Clinton that predated his campaign.
It's not clear how much impact O'Malley's nod will have. On the one hand, many state Democrats aligned with O'Malley we're already supporting Van Hollen. And O'Malley did not garner much support for his presidential bid even in his home state.
On the other hand, O'Malley is still one of the state's best-known names in Democratic politics, he was widely credited with running an energetic, issues-based presidential campaign, and he is still popular in many quarters of Baltimore and its suburbs -- exactly the geography that has become the battleground in the Senate race.
Van Hollen and Rep. Donna F. Edwards are running for the seat that will be left open next year by retiring Sen. Barbara A. Mikulski. The primary is tomorrow.