(CNN) -- Attorneys on both sides of the same-sex marriage debate faced off Monday in a federal appeals court in California, as a panel of judges heard arguments about the constitutionality of Proposition 8. In August, a federal judge ruled that the voter-approved measure, which defined marriage as a union between a man and a woman, violated the U.S. Constitution. The Ninth U.S. Circuit Court of Appeals considered an appeal of that judge's ruling Monday. The three-judge panel opened Monday's hearing with tough questioning of parties seeking to appeal the decision, including ProtectMarriage.com and Isabel Vargas, who's a deputy clerk and deputy commissioner of civil marriages for Imperial County, California, where voters overwhelmingly approved Proposition 8.
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NEW: Judges ask tough questions to side opposing same-sex marriage
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A panel of judges will hear arguments over whether the measure is constitutional
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A lengthy legal battle continues after voters approved Proposition 8
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A federal judge's earlier ruling says the proposition violates the Constitution
The state's high court had allowed same-sex marriage, but then the 2008 @placeholder voter referendum passed with 52 percent of the vote.