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Protesters on both sides of California Proposition 8, which would ban same-sex marriage, crowd the intersection of High Street and MacArthur Boulevard in Oakland, Calif., on Thursday, Oct. 23, 2008. (D. Ross Cameron/The Oakland Tribune)
SAN FRANCISCO — Leaders of the campaign to outlaw same-sex marriage in California are warning businesses that have given money to the state's largest gay rights group they will be publicly identified as opponents of traditional unions unless they contribute to the same-sex marriage ban, too.

ProtectMarriage.com, the umbrella group behind a ballot initiative that would overturn the California Supreme Court decision that legalized same-sex marriage, sent a certified letter this week asking companies to withdraw their support of Equality California, a nonprofit organization that is helping lead the campaign against Proposition 8.

"Make a donation of a like amount to ProtectMarriage.com which will help us correct this error," reads the letter. "Were you to elect not to donate comparably, it would be a clear indication that you are in opposition to traditional marriage. The names of any companies and organizations that choose not to donate in like manner to ProtectMarriage.com but have given to Equality California will be published."

The letter was signed by four members of the group's executive committee: campaign Chairman Ron Prentice; Edward Dolejsi, executive director of the California Catholic Conference; Mark Jansson, a member of The Church of Jesus Christ of Latter-day Saints; and Andrew Pugno, the lawyer for