Four Montana couples filed a lawsuit challenging the state’s ban on same-sex marriage on Wednesday.

The couples are represented by the American Civil Liberties Union of Montana (ACLU), argue that the constitutional ban violates their rights by denying them legal protections afforded to straight couples.

67% of Montana voters passed a constitutional amendment in 2004 banning same-sex marriage.

The complaint filed against the ban on Wednesday argues that it violates the couples’ right to equal protection under the Fourteenth Amendment.

Named under the lawsuit are Attorney General Tim Fox, a county clerk and state revenue director Michael Kadas.

Same-sex marriage is now legal in 18 states across the US, and a poll today found that support for it is at an all time high of 55%. There are currently only two states which have unchallenged same-sex marriage bans.

A judge struck down Pennsylvania’s ban on same-sex marriage yesterday, deeming the ban unconstitutional.

He wrote: “We are a better people than what these laws represent, and it is time to discard them into the ash heap of history.”

As well as the same-sex marriage lawsuit, the ACLU is also pursuing a separate complaint which seeks to have equal legal protections for same-sex couples in domestic partnerships.