A letter sent to Mormon bishops and signed by church president Thomas S. Monson and his two top counselors calls on Mormons to donate "means and time" to the ballot measure. A note on the letter dated June 20 says it should be read during church services on June 29, but the letter was published Saturday on several Web sites.
Church spokesman Scott Trotter said Monday that the letter was authentic. He declined further comment, saying the letter explains the church's reasons for getting involved.
The LDS church will work with a coalition of churches and other conservative groups that put the California Marriage Protection Act on the Nov. 4 ballot to assure its passage, the letter states.
In May, California's Supreme Court overturned a voter-approved ban on same-sex marriage, saying gays could not be denied marriage licenses.
"The church's teachings and position on this moral issue are unequivocal. Marriage between a man and a woman is ordained of God and the formation of families is central to the Creator's plan for His children," the four-paragraph letter states.
"We ask that you do all you can to support the proposed constitutional amendment by donating of your means and time to ensure that marriage in California is legally defined as being between a man and a woman," church leaders say in the letter. "Our best efforts are required to preserve the sacred institution of marriage."
California Mormons - there are more than 750,000, according to a church almanac - have heard and heeded similar calls from their leaders before.
In 2000, a letter from the pulpit asked members to give time and money in support of Proposition 22, a ballot measure prohibiting California from legally recognizing gay marriages performed outside the state. It passed but was later struck down by the courts.
The LDS church also fought same-sex marriage legislation in other states during the 1990s. As recently as 2006, it signed a letter to Congress seeking an amendment to the U.S. Constitution that would define marriage as being between a man and a woman.