A multi-million-dollar advertising campaign asking people to “open their hearts and minds” to same-sex marriage is due to launch in five Californian cities this week.

The 60-second ads will run from Thursday in Sacramento, San Francisco, Los Angeles, San Diego and Palm Springs as part of a month long campaign to encourage open discussion about same-sex marriage.

“The long-term goal is to have the majority of Californians support the freedom to marry – to change the climate here,” said Geoff Kors, executive director of Equality California, which is coordinating the campaign.

The ad depicts a traditional wedding, with an excited crowd, a flower girl tossing petals and a tuxedoed groom.

As the bride walks down the aisle, she is tripped by a spectator and sprawls onto the floor.

The tagline reads: “What if you couldn’t marry the person you loved? Every day gay and lesbian couples are prevented from marrying. Support the freedom to marry.”

Benjamin Lopez, spokesman for the Traditional Values Coalition, which opposes same-sex marriage, said the advertising campaign was “grasping at straws.”

“This notion of a redefined marriage goes against the laws of God and against nature,” he said, according to the Sacramento Bee.

Supplementing the TV campaign, thousands of volunteers are expected to participate in the campaign, entitled “Let California Ring,” by conducting house parties, knocking on voters’ doors, giving speeches or assisting in e-mail or web campaigning.

In the next few days Governor Arnold Schwarzenegger is expected to veto a measure the state Legislature passed last month that would allow same-sex marriages in California.

He vetoed a similar law two years ago, saying he was upholding “the will of the people,” a referemce to the landslide victory of Proposition 22, the 2000 ballot measure that says “only marriage between a man and woman is valid and recognised in California.”

The constitutionality of California’s ban on same-sex marriage is being challenged in the state Supreme Court.

Same-sex couples who register as domestic partners are entitled to virtually every right and responsibility granted by the state to spouses.

But California’s domestic partnerships are not necessarily recognised in other states, nor do they qualify for Social Security, veterans and other federal spousal benefits.

Even if marriage and domestic partnerships were separate but equal, such a dual system would be unfair, according to Assemblyman Mark Leno, who proposed this year’s same-sex marriage legislation.

“To deny any citizen the fundamental right to marry the person that he or she loves is wrong, and in my opinion, un-American,” Leno told the Bee

The campaign’s goal is to spark a million conversations about same-sex marriage and to prod an additional 500,000 Californians to support legalisation.

The logo for “Let California Ring” features a wedding band as the “O” in the state’s name. Supporters will be urged to purchase a ring through the campaign and wear it to show support and spark conversation.

Garry South, a Democratic political strategist, said he wishes the new campaign well but doubts that opponents of same-sex marriage will change their minds through advertising or door-knocking.

“It’s like trying to change people’s minds about abortion by putting ads on the air. I don’t think it works,” South said.

“This campaign is about changing the climate in California around this issue,” Seth Kilbourn, Equality California’s political and policy director, told the San Francisco Chronicle.

He referenced polls showing Californians almost evenly split on the issue.

A poll in 2000 by the Policy Institute of California found that voters opposed same-sex marriage 55 to 38 per cent; a poll by the same group last year showed a major shift: 48 to 46 per cent.

This campaign is intended to move the state “over the tipping point,” Kilbourn said.

“The homosexual community continues to attempt to attract the emotions of society,” Ron Prentice, executive director of the California Family Council, part of an anti-gay marriage consortium called ProtectMarriage.com, told the Chronicle.

“Our hope is that society will recognise that the meaning and purpose of marriage is more than an emotional argument.”

The campaign has received $400,000 (£200,000) from Jim Hormel, the San Francisco philanthropist and former U.S ambassador to Luxembourg; a $500,000 commitment from the Evelyn and Walter Haas, Jr. Fund; and $500,000 from an anonymous donor.

The campaign’s Web site is http://www.letcaliforniaring.org

The TV commercial can be viewed at http://tinyurl.com/27qu8o