Brad Pitt has donated $100,000 to the campaign to defeat Proposition 8, the California ballot initiative that would ban same-sex marriage in the state.

His contribution, the largest so far from a prominent individual in the entertainment business, comes as fund-raising to defeat the measure lags behind that of supporters. Polls suggest the proposition will have a tough time winning a majority, but there has been considerable speculation that dynamics could change once ads are launched in earnest in the coming weeks.

“Because no one has the right to deny another their life even though they disagree with it, because everyone has the right to live the life they so desire if it doesn’t harm another and because discrimination has no place in America, my vote will be for equality and against Proposition 8,” Pitt said in a statement.

Jennifer Kerns, a spokeswoman for ProtectMarriage, one of the groups campaigning for passage of Proposition 8, said of Pitt, “We certainly respect his right in the political process, but we don’t believe he is the most exemplary figure on the issue of marriage.”

Pitt’s involvement adds a superstar name to an effort that will be the focus of nationwide attention when voters go to the polls Nov. 4. Should the proposition fail, it would mean the same-sex marriages that started in June after a state Supreme Court ruling could continue. Should the measure pass, it would add to the state constitution language making marriage legal only between a man and a woman. It’s not certain if same-sex couples who married this year would be grandfathered in should a ban be put in place.

Pitt’s contribution, to the group Californians Against Eliminating Basic Rights, is seen as a challenge to others in the industry to step up their fund-raising efforts — no easy task as politicians of all stripes court donors in the run-up to the election. In addition to Pitt, others who have chipped in so far include David Geffen, who has donated $50,000, and CAA’s Bryan Lourd, who contributed $5,000.

But in the latest issue of IN Los Angeles magazine, which covers the local gay community, writer Karen Ocamb reports on a handful of openly gay entertainment figures who have yet to donate, including Ellen DeGeneres, Rosie O’Donnell, Elton John and Melissa Etheridge, according to state fund-raising records.

The largest individual donation so far to the “No” campaign has come from philanthropist David Bohnett, who contributed $600,000, although LGBT rights orgs have contributed substantially more. The Human Rights Campaign has contributed $2 million, Ocamb wrote.

Many more donors are expected in October, when Ron Burkle plans to host a major event for the No on 8 campaign at his Greenacres estate in Beverly Hills.

Even as statewide polling shows a majority of residents favor gay marriage, supporters of same-sex nuptials are being outraised by their counterparts. No on 8 groups have raised $11 million, but supporters of a same-sex marriage ban have collected more than $16 million. Groups such as Focus on the Family and the Knights of Columbus have given substantial sums to the campaign seeking passage, although contributions from entertainment businesses are scant. The Pat Boone Foundation has given $300.

“It is our hope that others in the entertainment industry will step up and match Brad Pitt’s heroic commitment to equality and to defeating Prop. 8,” said political strategist Chad Griffin, who is working to defeat the measure.

Pitt has participated in campaigns for such state propositions as a stem cell initiative and a renewable energy initiative, but unlike in those efforts, he is making a substantial monetary contribution to the No on 8 campaign.

“I think Brad sees this as a human rights issue and an issue of justice,” said political consultant Trevor Neilson, who represents Pitt and Angelina Jolie on their political and philanthropic efforts.

Neilson added, “I would say the entertainment community needs to step up to the plate and fight this.”

Pitt has been on the record in support of same-sex marriage before. Asked by Esquire in 2006 about his partnership with Jolie, he said, “Angie and I will consider tying the knot when everyone else in the country who wants to be married is legally able.”

Gov. Arnold Schwarzenegger opposes Proposition 8, as does Democratic nominee Barack Obama. Republican nominee John McCain supports it.