If Chief U.S. District Judge Vaughn Walker decides against a trial, some issues will be taken off the table, including any examination of the Prop. 8 campaign, its ads and ballot arguments, and the motives of its backers.

That could prove to be the critical issue in the case. The plaintiffs hope to prove that Prop. 8, promoted as a measure to preserve traditional marriage, was actually motivated by its backers' moral and religious disapproval of homosexuality. That's not a legitimate basis for a state law, the U.S. Supreme Court ruled in a Colorado case in 1996.

In addition, any ruling without a trial would carry little weight with higher courts, which are required to defer to Walker's factual findings but not his legal conclusions.