Conservative US Christians see California's gay marriage issue as a defining battle that could set the stage for a national showdown and get the vote out for the Republican Party in the November presidential election, a leading evangelical said.

"People feel like this California fight is for all the marbles," Dr Richard Land, president of the Southern Baptist Convention's Ethics & Religious Liberty Commission, told Reuters at the SBC's annual meeting in Indianapolis.

The 16 million-strong SBC is America's largest Protestant denomination and is a key part of the Republican Party's conservative Christian base.

California is set for a pitched political battle after it was announced last week that the November ballot would include a constitutional amendment limiting marriage to unions between men and women. A similar proposal is on the Florida ballot.

Initiatives to ban gay marriage played a role in US President George W Bush's 2004 re-election as they brought Republican conservative religious voters to the polls.

If a majority of California voters approve the measure, it would neutralise last month's state Supreme Court ruling that said preventing same-sex couples from marrying was unconstitutional and discriminatory. The state is set to begin marrying gay couples next week.

Land said the court decision was bad timing for Democratic presidential contender Barack Obama, who will face Republican John McCain in the November presidential election, because it would serve as a red flag to self-styled "values voters" who lean heavily Republican.

"I would say that if I were the Obama campaign, I would not be very happy with the California Supreme Court because it has put back on the front burner an issue that is not good news for Democratic nominees," said Land, who is regarded as one of the most influential conservative evangelicals in America.

"My guess is that the same-sex marriage issue being on the ballot puts California in play in McCain versus Obama," he said.

Land said various denominations and religious groups, including Mormons and Catholics, already were joining forces in California with evangelicals to defeat gay marriage.

"If California rejects their Supreme Court and changes their constitution, that's going to be huge...if we win in California we have a chance of winning this nationwide," he said.

No Republican presidential candidate has won California since George HW Bush in 1988. The most recent state polls show Obama leading McCain, generally by double digits, but most of those were conducted before the court ruling.

California, America's largest state and often a trend-setter for the rest of the country, will wed any gay couple, regardless of where they live. By contrast, Massachusetts, the other US state where same-sex couples can marry, only weds residents or those from a neighbouring state.

Land said this would put the issue on the evangelical radar screen outside of California.

"You're going to have same-sex couples getting married in California and coming back to all the states in the country and demanding that their marriages be recognised," he said. "So it's going to be an issue in Cedar Rapids, in Dallas, in El Paso."

California's size means it has the most electoral college votes up for grabs in the presidential race and the candidate who gets the most popular votes in a state wins all its electoral votes.

Many evangelical Christians, such as Southern Baptists, regard gay marriage as a "make or break" issue that threatens the traditional family and by extension their vision of a functioning society. They also see same-sex relations in general as sinful.