From the WSJ law blog, detailing the three similar court challenges to Prop 8. They hinge on whether Prop 8 is an amendment or a revision to the state constitution. The difference is crucial.
The three lawsuits challenge the procedure by which the referendum was passed. Under California law, there are two categories of changes that can be made to the state constitution: amendments and revisions. Amendments are more minor changes; revisions are larger in effect. This is important because each has its own process for taking effect — essentially different ways they go before the voters. An amendment can go in the form of a ballot initiative, which requires a certain number of signatures to make its way on. Constitutional revisions, however, have to have a two-thirds blessing from each house of the state legislature to make the ballot.
But there was no such vote. Perhaps because Yes on 8 knew they would lose.
Now, the problem, at least from the point of view of Prop. 8 supporters, is that the legislature had previously indicated a willingness to support same-sex marriage. So the proposition’s supporters were unwilling to treat this [change] as a revision and send it to the legislature, opting instead to treat it as an amendment. The Prop. 8 opponents are arguing that this change actually constitutes a revision, not an amendment, and therefore needed to go through the legislature.
I’m not a lawyer but it seems this challenge has a reasonable chance of succeeding. San Francisco, the County of Los Angeles, the City of Los Angeles, and Santa Clara County among others have joined forces on one of the lawsuits. This turns the tables a bit. LDS may have been able to raise millions of dollars, but now they face battalions of lawyers on the opposing side.