The bill passed the state Senate last Wednesday, 25-22.
Opponents are already mobilizing to get a referendum on the ballot in November. They’ll need 55,736 signatures to do so.
Martin O’Malley has championed gay-marriage legislation, speaking at rallies with marriage-equality supporters where he has vowed to sign the bill once it passes.
His wife even called anti-gay lawmakers “cowards,” though she ended up apologizing for that.
Maryland will become the eight state to legalize same-sex unions, joining Washington, New York, Iowa, New Hampshire, Vermont, Massachusetts, and the District of Columbia.