Gay-rights advocates are preparing a lawsuit alleging that the state is violating the equal protection rights of same-sex couples by not passing a civil-unions law.

Lambda Legal and the American Civil Liberties Union of Hawai'i yesterday said they would bring the lawsuit after the state House voted on Friday to indefinitely postpone action on civil unions this session.

The lawsuit will be based on a 1993 Hawai'i Supreme Court ruling which held that denying same-sex couples the ability to marry was a violation of their equal-protection rights under the state Constitution.

Voters approved a constitutional amendment in 1998 that gave the state Legislature the right to define marriage — and lawmakers have defined it as between a man and a woman — so gay-rights advocates are unable to claim a right to marry.

Instead, they will claim that the state's decision not to extend equal rights and benefits to same-sex couples is a violation of their equal protection rights. They will also argue that the state's reciprocal beneficiaries law, which gives same-sex partners some rights, is inadequate.

"Our legislators are bound to uphold the Hawai'i Constitution, and clearly they have failed," said Alan Spector, legislative committee co-chairman for Equality Hawai'i, which favors civil unions.

"We will continue to work with them in the hopes that they will do the right thing, but we are also bound to pursue all options before us," Spector said

State Attorney General Mark Bennett said he does not believe the state's laws on the matter are unconstitutional.

The state Senate passed a civil-unions bill in January that gives same-sex and heterosexual couples the ability to enter civil unions and receive the same rights, benefits and responsibilities as marriage under state law.

The state House, which passed a civil-unions bill last session that applies only to same-sex couples, declined to move forward on the Senate version of the bill because leaders could not muster a two-thirds' majority to override a potential veto.

"Call it what you want, our belief is that the Hawai'i Constitution still requires the extension of equal rights and benefits," said Lois Perrin, the legal director of the ACLU Hawai'i.

New York-based Lambda Legal is a group that works for equality and civil rights for lesbian, gay, bisexual, and transgender people.