The groups, ProtectMarriage.com and VoteYesMarriage.com, have filed ballot language with the California secretary of state that would, if approved by voters, amend the California Constitution to limit marriage to one man and one woman regardless of how the Supreme Court rules.
They have until late April to gather signatures from 694,354 voters to qualify the measures for the November election and have begun circulating petitions, fundraising and recruiting volunteers through Southern California churches.
The Supreme Court has scheduled a March 4 hearing for oral arguments in four cases brought by 15 same-sex couples, the city of San Francisco and a gay rights group challenging state laws limiting marriage to opposite-sex couples.
The lawsuits grew out of the monthlong wedding spree that took place in San Francisco four years ago when Mayor Gavin Newsom directed employees to grant marriage licenses to gay and lesbian couples, an act of civil disobedience eventually halted by the high court.
Although the groups opposed to same-sex marriage launched marriage amendment campaigns in 2005, they fell far short of collecting enough signatures.
There is much more money and organization behind their efforts now, say advocates on both sides of the issue.
"The very fact that this is in front of the Supreme Court, I think, will highlight the need for voters to take the issue away from the courts, period," said Andrew Pugno, a lawyer for ProtectMarriage.com.
By enshrining the two laws that already prevent gays from marrying in the state constitution, both amendments would overrule the justices if they decide the current statutes are an unconstitutional violation of the civil rights of same-sex couples. A ruling is expected in June.
The VoteYesMarriage initiative would go a step further, however, by prohibiting the state from granting gays the spousal rights and tax benefits of marriage, as it already has by allowing gays to register as domestic partners.
If the initiative passes, those rights would be eliminated.
As in 2005, the philosophical differences between the two groups has them competing for support from the same voters and conservative activists, as well as facing the prospect of a divisive campaign if both measures make the ballot.
Lined up behind ProtectMarriage are the widow and political allies of the late state Sen. William J. "Pete" Knight, who sponsored a 2000 ballot initiative approved by voters that strengthened the state's ban on gay unions.
Proposition 22 is one of the laws that will be before the Supreme Court next month.
According to campaign finance reports, the group last year raised just under $130,000 in donations, most of it from Focus on the Family, the Colorado Springs-based evangelical organization founded by Dr. James Dobson, and from the National Organization for Marriage, a Princeton, N.J.-based group established to fight gay marriage in that state.
The money has helped ProtectMarriage hire paid signature gatherers to supplement the volunteers being recruited by a coalition of pastors in San Diego County formed to back the amendment.
VoteYesMarriage, which is headed by veteran activist Randy Thomasson, raised $84,000 last year, some of which it has spent placing ads in religious newspapers urging readers to help their pastors "know the difference between a true and a false marriage amendment."
Both groups say they have poll results showing that only their amendment would be approved at the ballot box.
So far, no state court has joined Massachusetts in recognizing marriage as a civil right. Along with California, Connecticut and Iowa also have cases pending on whether allowing gays to enter into civil unions and domestic partnerships provides a legal equivalent to getting married.
Gay marriage proponents are gearing up for a fierce and expensive fight, said Geoffrey Kors, executive director of the gay rights group Equality California. With the court case pending, it opted not to ask the Legislature this year to pass a bill legalizing same-sex marriage, as it has done twice before only to have the measures vetoed by Gov. Arnold Schwarzenegger.
While 26 states have amended their constitutions to ban same-sex marriage, both opponents and supporters recognize California, home to more same-sex couples than any other state and historically a gay rights pioneer, as a crucial win or loss in their column, Kors said.
If one or both of the amendments wind up on the November ballot, "this will be an election like we haven't seen before on this issue," he said.