Maria Shriver, the wife of Governor of California Arnold Schwarzenegger, has publicly expressed her support for same-sex marriage in the state.
A ballot measure on election day, November 4th, seeks to ban gay and lesbians from getting married.
Proposition 8 has the support of church groups in California and many Republican politicians.
“I believe in people’s right to choose a partner that they love, and that’s a decision that I have come to, and I have felt that way for a long time,” said Ms Shriver, a niece of former President John F Kennedy.
Her husband has also publicly opposed Proposition 8.
Computer giant Apple has declared Prop 8 a civil rights issue and donated $100,000 to the campaign to defeat it.
Google has also publicly backed gay marriage.
“We strongly believe that a person’s fundamental rights — including the right to marry — should not be affected by their sexual orientation,” Apple said in a statement.
Last week a Public Policy Institute of California found Proposition 8 losing 52% to 44% among likely voters.
4% are undecided. In polls taken in August and September by the institute found double digit leads for opponents of the measure.
Proposition 8 would amend the state Constitution to “eliminate right of same-sex couples to marry.”
In May the California Supreme Court overturned a ban on same-sex marriages in the state.
The Court voted 4 to 3 to strike down the ban.
Opponents of gay marriage raised more than a million signatures to place the initiative on the November ballot.