OLYMPIA, Wash. — As lawmakers held their first public hearing on legalizing same-sex marriage, a previously undecided Democratic senator on Monday announced her support for the measure, all but ensuring that Washington will become the seventh state to allow gay and lesbian couples to get married.

The announcement by Sen. Mary Margaret Haugen, D-Camano Island, that she would cast the 25th and deciding vote in favor of the issue came as hundreds of people filled the Capitol to advocate for and against gay marriage.