US opponents of gay marriage pushed their campaign on both coasts this week, setting it up as a potential hot-button issue in the November 4 elections around the country.

Activists said on Tuesday California was set for its biggest political battle in years after it was announced the November ballot would include a constitutional amendment limiting marriage to unions between men and women.

A similar measure also is on the November ballot in Florida, which is likely to be closely fought in the election, and there also are national implications.

"Values voters have even more reason to go to the polls in 2008," said Tony Perkins, a leading religious conservative who is president of the Family Research Council.

Initiatives to ban gay marriage played a role in President George W Bush's 2004 re-election as they propelled the Republican Party's conservative Christian base to the polls.

In New York, conservative lawmakers were among those who filed a suit against Governor David Paterson on Tuesday, charging that he overstepped his authority by ordering state agencies to recognise out-of-state same-sex marriages.

California is scheduled to begin marrying gay couples later this month but until then, Massachusetts is the only state that allows same-sex marriage. More than 25 states have constitutional amendments barring gay marriage.

Although Republican presidential candidate John McCain is viewed by many religious conservatives as soft on core values issues, gay marriage could affect turnout where it is on the ballot.

"Which will actually be a larger draw in California? Will the presidential choice motivate people or will the issue of marriage motivate people?" said Ron Prentice, chairman of the Protect Marriage Coalition, a group of 31 organisations that are sponsoring the measure in California.

"We believe that this measure on the ballot will actually bring significantly more people to vote," he said.

BATTLE IN CALIFORNIA

If more than half 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.

"This will be the biggest battle on the West Coast since the recall," Randy Thomasson, president of the Campaign for Children and Families in reference to the emotional 2003 election that recalled then-California Governor Gray Davis.

Gay marriage opponents are urging county officials to practise civil disobedience by refusing to marry same-sex couples this month when the marriages are scheduled to begin.

The group behind the California ballot measure expects to spend at least $10 million campaigning for it.

Gay marriage proponents have said they will match them dollar-for-dollar. "We believe that the time has passed that an amendment like this could survive at the ballot box," said Kate Kendall of the National Center for Lesbian Rights.

Polls offer conflicting predictions. A Field poll published last week showed 51 per cent in favour of gay marriage with 42 per cent opposed. But a Los Angeles Times poll the previous week found 54 per cent backed the amendment defining marriage as between a man and a woman with 35 per cent opposed.

Evangelical Protestants, who account for one in four American adults, are especially keen to prohibit gay marriage, which they view as a threat to the traditional family.