A federal appeals court Tuesday rejected an attempt by conservative groups and state lawmakers to stop gay marriage (search) in Massachusetts.

The Florida-based Liberty Counsel (search), which launched the lawsuit, said it would appeal to the U.S. Supreme Court.

The plaintiffs had argued that Massachusetts' high court usurped the power of the Legislature — and thereby violated the U.S. Constitution — when it ruled last year that gay couples are entitled to wed.

The 1st U.S. Circuit Court of Appeals disagreed Tuesday, and said the appropriate way to contest the state court ruling is by amending the Massachusetts Constitution — a long process already under way.

The lawsuit was filed on behalf of Robert Largess, the vice president of the Catholic Action League (search), and 11 state lawmakers.

The federal appeals court had rejected an earlier request by the groups to stop the first marriage licenses from being issued to gay couples in May.

Tuesday's ruling was the latest in a series of defeats for conservative groups in both state and federal courts.

The appeals court said the November 2003 gay-marriage ruling by Massachusetts' Supreme Judicial Court did not violate a clause in the U.S. Constitution that calls for a republican form of government in each state.

The only way a state could run afoul of the clause would be to establish a monarchy or take other action that clearly deviated from the republican form of government, the appeals court said.