"We knew it was a close election from the get-go," U.S. Sen. Ben Cardin said from Brown's headquarters in College Park.
At Hogan headquarters in Annapolis, supporters posted a map of Maryland, overwhelmingly colored in red for the rural and Eastern Shore jurisdictions Hogan carried in early returns.
"It's not a realignment. It's not turning a blue state red. It's people who are tired of the last eight years," Hogan spokesman Adam Dubitsky said.
Brown led in the Washington-Baltimore region in early voting returns.
A win by either candidate would offer a historic distinction. Brown would be Maryland's first African-American governor. Hogan would be only the second Republican governor elected since the 1960s in deep blue Maryland.
At Brown's campaign headquarters in College Park, Riggs Alumni Auditorium was buzzing with about 50 Brown supporters standing in clutches, chatting and waiting for some results.
Hogan had a nine-piece, live band entertaining the crowd at his Annapolis headquarters.