"We need someone who can hit the ground running," Kamenetz said at an event in Randallstown. "Chris has paid his dues."
The endorsement was not a huge surprise, but it does underscore that Van Hollen has managed to lock up support from many of the state's most significant political leaders -- now including the executives of the state's three largest counties.
Baltimore County in particular could be an important battleground in the race for the Democratic nomination, which also features Rep. Donna F. Edwards. Polls have shown Edwards winning in her home base of Prince George's County and Van Hollen taking Montgomery County. With the Washington suburbs split, the Baltimore region becomes the next-highest concentration of Democrats.
A Sun/UB poll over the weekend showed Edwards with a significant lead in the Baltimore region after several other surveys suggested a close race.
The primary election is April 26.
Kamenetz has been rumored to be interested in higher office himself, and his campaign announcement for Monday's event listed a new address in the Locust Point community in South Baltimore. His annual campaign finance report in January showed he had more than $1.3 million cash on hand.