The Maryland chapter of the AFL-CIO labor union has endorsed an effort to legalize gay marriage in the state, The Baltimore Sun reported.

The 300,000-member union approved last month a resolution in support of the soon-to-be introduced legislation. The group will announce details at a press conference to be held on Thursday.

Smaller unions have already joined the coalition of groups lobbying for the bill's passage.

A gay marriage bill ran aground last year in the House of Delegates after it narrowly cleared the Senate.

Democratic Governor Martin O'Malley has pledged to shepherd the bill through the Legislature during the upcoming legislative session, which opens next week.

Opponents of marriage equality have formed the Maryland Marriage Alliance to lobby against passage of the bill. The coalition is supported by the Maryland Catholic Conference and the National Organization for Marriage (NOM), two groups which strongly opposed the legislation last year.

Lawmakers in nearby New York approved a gay marriage bill since Maryland's defeat last spring, possibly boosting the bill's chances of passage this year.

(Related: Gov. Chris Gregoire to introduce gay marriage bill in Washington State.)