Steven Spielberg and his wife, Kate Capshaw, are donating $100,000 to the campaign to defeat a proposed constitutional ban on same-sex marriage in California.

Their contribution is the latest high-profile donation to the campaign, as gay marriage advocates seek to narrow a gap in fund-raising with supporters of Proposition 8. Brad Pitt announced last week that he would donate $100,000 to the “No on 8” campaign, in what was seen as a challenge to other industry figures to pony up more for the fight.

The proposition would restrict marriage to unions between a man and a woman.

The couple said in a statement, “By writing discrimination into our state constitution, Proposition 8 seeks to eliminate the right of each and every citizen in our state to marry regardless of sexual orientation. Such discrimination has NO place in California’s constitution, or any other.”

Producer Bruce Cohen, who has been working on the campaign to defeat Proposition 8, said in a statement, “It’s phenomenal that Kate and Steven are affirming their unwavering commitment to equality in such a significant a way.”

Cohen was married in June to Gabriel Catone in a ceremony performed by Los Angeles Mayor Antonio Villaraigosa. A recent Field Poll shows that the proposition is losing support, with 55% of voters indicating that they would say no to the proposed amendment and 38% saying they would support it.

But opponents of the proposition have been out-raised in funding and worry that gay-rights advocates will grow complacent. “Yes on 8” supporters have collected $16.2 million from groups ranging from Focus on the Family to the Knights of Columbus, compared with $10.8 million raised by “No on 8” groups. The L.A. Weekly reported last week that only $1.8 million in contributions for the “No on 8” campaign had come from L.A. County.

“Support from the entertainment industry is critical to defeating Prop 8. The forces behind this measure are highly motivated and supported by millions of out-of-state dollars,” said political strategist Chad Griffin, who is working to defeat the initiative.

San Francisco Mayor Gavin Newsom, San Francisco City Attorney Dennis Herrera and New York Gov. David Paterson are co-hosting a fund-raiser for “No on 8” on Thursday, and Ron Burkle plans to host an event at his Beverly Hills estate sometime in October.

The “No on 8” campaign unveiled the first of its TV ads Monday, featuring a straight couple who have been wedded for 46 years who talk about their gay daughter losing her right to marry.

A spokeswoman for “Yes on 8” could not be reached. But after Pitt donated last week, “Yes on 8” campaign manager Karen England released a fund-raising e-mail contending that his contribution was “more evidence that Hollywood’s elite are out of touch with the average American.”

She wrote, “With wealthy Hollywood lining up to destroy marriage, we call on every concerned citizen to donate $15 for every Brad Pitt movie they have ever seen. And if they haven’t seen any Brad Pitt movies, to donate $10 for those unseen films.”