The legislation, which will go to a vote in the state House of Representatives next week, seeks to redefine marriage as the legal union of two people rather than between a man and a women. It passed the Senate by a 20-15 margin.
Maine Governor John Baldacci once opposed gay marriage, but said earlier in April he is keeping an open mind on the issue.
Approval in the Democratic-controlled Senate of the rural state of 1.3 million people underlines a concerted push for same-sex marriage recognition in New England's six states by gay and lesbian advocates.
Gay & Lesbian Advocates & Defenders, a group of lawyers who led the legal fight for same-sex marriage in Massachusetts and Connecticut, has set a target of bringing same-sex marriage to all New England states by 2012.
In November, Connecticut became the second state to allow legal same-sex weddings after neighboring Massachusetts' top court ruled in 2003 that a ban on gay marriage was unconstitutional, paving the way for the first same-sex marriages in the United States the following year.
In a single week in April, Iowa and Vermont also legalized same sex marriage. And on Wednesday, New Hampshire's state Senate approved a gay marriage bill, about a month after its House approved it. The bill needs New Hampshire Governor John Lynch's signature to become law.
"With progress in New Hampshire and a win in Vermont, winning in Maine could put us only one state away from our goal," Lee Swislow, executive director of Gay & Lesbian Advocates & Defenders, said in a statement.
Gay marriage legislation has yet to advance in Rhode Island.
Some economists say carving out an economic niche for gay and lesbian weddings -- and the spending that comes with them -- makes sense at a time same-sex marriage has stalled in California and a recession is deepening.
Keeps on rollin rollin rollin!!