Why? It's not exactly clear.
Brown's campaign said Messina will also make a public announcement about the race, though Brown's campaign manager Justin Schall was mum about about what that announcement could be.
Messina oversaw Obama's 2012 re-election campaign, and worked in the White House as a deputy chief of staff during the president's first term. This year, he led the Organizing for America's so-far-unsuccessful push for tougher federal gun laws.
Brown is the only Democrat to formally annouce for next June's primary to suceed term-limited Gov. Martin O'Malley, also a Democrat.
Two other Democrats have signaled they plan to run. Attorney General Douglas F. Gansler said he plans to announce in September. Montgomery County Del. Heather Mizeur told her supporters in a Wednesday email that her campaign reached an important fundraising goal that would determine whether her potential bid for governor will continue.
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