SAN FRANCISCO (Reuters) - The biggest U.S. gay rights battle next year is brewing in a California federal court as raucous fights over same-sex marriage in state legislatures and at state ballot boxes subside.

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In part, 2010 will reflect a growing move by same-sex marriage advocates to building support for their civil rights cause outside of the election process.

The federal challenge to California's ban may be the only conflict in clear sight after a mixed 2009 that saw Iowa, Vermont, New Hampshire legalize gay marriage and Washington, D.C., vote for legalization, while there were setbacks in other states that had been expected to follow.

"The focus is very much on this one case," said Andy Pugno, a California lawyer who successfully defended California's ban in the state supreme court and is helping in the federal defense as well.

New York state legislators failed to back gay marriage and a New Jersey effort has hit snags and has a few weeks to act before a new governor who opposes such gay unions takes office. Maine voters rejected same-sex marriage by a thin margin similar to the California 2008 ban, which is being contested in the San Francisco federal court.

"We believed that this is something that needs to be vindicated at the federal constitutional level, and I think that that is reinforced by what's happened in Maine and what did not happen, for example in New Jersey, and what did not happen in New York," said David Boies, one of the lead lawyers in the federal case.

Massachusetts legalized gay marriage in 2004 and California had a summer of legalization in 2008 before voters banned it.

Trial is set to start January 11 in San Francisco. Boies and co-lead Ted Olson argue that marriage is a U.S. constitutional right too fundamental to limit and that gays and lesbians are a discriminated group that deserve special court protection.

Opposing attorney Charles Cooper says restricting marriage to a man and a woman reflects a reasonable government position that heterosexual couples are best for families. It is not a question of hate, and gays and lesbians have plenty of political power, making special court protection unnecessary....