The David Boies-Ted Olson lawsuit challenging Proposition 8 gained a new group of supporters this evening: The ACLU, Lambda Legal and the National Center for Lesbian Rights filed an amicus brief supporting the federal challenge.

When the suit was filed last month, the ACLU and Lambda Legal were among a coalition of groups that issued a joint statement expressing their reluctance, fearing that “bad rulings” at the federal level could set back the drive for same-sex marriage.

It was unclear why they apparently had a change of heart.

In their brief, filed in U.S. District Court in San Francisco, the org said that Prop 8 “violates the federal guarantees of equal protection and due process” and “denies same-sex couples the right to marry in a unique historical context in which the denial can only be deemed a declaration of inequality.”

The three groups led the legal fight against Proposition 8 before the state Supreme Court, which upheld the initiative but also the marriages that have been performed.

Chad Griffin, president of the American Foundation for Equal Rights, which is backing the federal challenge, said in a statement: “This case is about the denial of fundamental Constitutional rights, and its significance is underscored by the united front presented by the coalition backing this case.”