A national gay rights advocacy group took its state-by-state fight for same-sex marriage equality to a federal court for the first time Tuesday, filing a lawsuit in a bid to force Nevada to allow gay and lesbian couples to marry.

The Lambda Legal Defense and Education Fund lawsuit focused on the differences between rights granted by a domestic partnership law passed by the state Legislature in 2009 and a gay marriage ban made part of the state constitution by voters in 2002.

It seeks to void the constitutional ban "and any other sources of state law that exclude same-sex couples from marriage," and to recover unspecified legal costs and fees. It claims eight same-sex couples are being discriminated against because they aren't allowed to marry.

"Nevada already gives same-sex couples the rights and responsibilities of family, parenting and relationships," said Tara Borelli, a Los Angeles-based Lambda Legal attorney. "What is the reason for not giving them the one word that says it all?"

The civil lawsuit filed in U.S. District Court in Las Vegas names Republican Gov. Brian Sandoval as a defendant. It alleges state lawmakers established a "selective bar to access to marriage" with the 2009 domestic partnership law that was passed over a veto by then-Gov. Jim Gibbons, a Republican.