* Judge found banning gay marriage unconstitutional
* Most believe case headed to U.S. Supreme Court (Adds pro-Proposition 8 comments, paragraph 11)
By Dan Levine
SAN FRANCISCO, Aug 5 (Reuters) - One day after a federal judge struck down California's ban on gay marriage, supporters of the voter-approved law on Thursday notified the court they would appeal, firing a new salvo in what experts say will be a long legal battle.
The one-paragraph document, which informs U.S. District Court Judge Vaughn Walker that the defendants intend to appeal his decision to the 9th U.S. Circuit Court of Appeals.
That appeal had been expected in a politically charged case that most believe will ultimately be decided by the U.S. Supreme Court and could inject a divisive social issue into U.S. congressional and state elections this fall, including California's race for governor.
Walker overturned California's voter-approved ban on gay marriages, known as Proposition 8, on Wednesday after finding that it violates the due process and equal protection provisions of the U.S. Constitution.
But the judge ordered that Proposition 8 remain in place at least temporarily while he decides on a request by supporters of the law to keep it intact as it moves to a higher court.
Walker issued his legal "stay" in order to give both sides a chance to file legal papers on whether the halt should be extended during the appeals process.
PLAINTIFFS WILL OPPOSE STAY
The deadline for each side to issue legal briefs is Friday, and Walker could rule on an extension at any time after that.
Theodore Boutrous Jr. of the law firm Gibson Dunn & Crutcher, which represents same-sex couples in the case, told Reuters that "our plan is to oppose a stay."
Even if Walker decides to lift the stay, an appeal to the 9th Circuit by the measure's supporters would put the judge's ruling on hold, meaning the battle is expected to continue for months to come, experts said.
Boutrous said plaintiffs intend to ask the appeals court to hear the case on a fast schedule.
Jim Campbell, an attorney with pro-Proposition 8 Alliance Defense Fund, said supporters have not yet decided whether they would agree to a faster schedule, and he estimated that initial legal briefs defending the law could take two to three months to file. Oral arguments might not begin for another year.
Meanwhile, the issue is certain to make the agenda of many congressional and state elections in the fall, which is exactly what Democrats and President Barack Obama, who has said he opposes legalizing gay marriage, had hoped to avoid.
Already in California, Republican Meg Whitman and Democrat Jerry Brown, who are running for governor, released statements staking out essentially opposite sides of the issue, with Brown in favor of the ruling and Whitman opposed.
Other challenges are under way across the United States, where a 2009 Gallup poll found that 57 percent of Americans oppose same-sex marriage.
In Massachusetts, a judge recently struck down the Defense of Marriage Act, which defines marriage as the union of one man and one woman under U.S. law.
Same-sex marriage is currently legal in Massachusetts, Connecticut, Iowa, Vermont, New Hampshire and Washington, D.C.
In July, Argentina became the first South American country to legalize same-sex marriage. A few other countries around the world permit it, including the Netherlands, Sweden, Portugal and Canada. (Additional reporting by Dan Whitcomb; Editing by Bob Tourtellotte and Eric Walsh)