Maryland Gov. Martin O'Malley will sign a same-sex marriage bill Thursday afternoon, making the state the eighth to legalize gay nuptials.
The earliest it could become effective is January, but the law still faces an impending hurdle.
Opponents of same-sex marriage have begun organizing to collect signatures for a referendum petition to place the law on the November ballot.
Opponents need 56,000 signatures by June 30 -- but have said they expect to collect almost twice that, the Associated Press reported.
Maryland's House of Delegates approved the legislation Feb. 17 on a 72-to-67 vote and sent it to the Senate, which approved it six days later on a 25-to-22 vote.