Proposition 8, the constitutional amendment that would ban same-sex marriage in California, is opposed by 51 percent of likely voters with 42 percent in favor, according to a new Field Poll.

Those results put the proposed ban in a politically perilous position in the Nov. 4 election, said Mark DiCamillo, director of the nonpartisan Field Poll.

“Starting out behind is usually an ominous sign for a proposition,†DiCamillo said. “Over 90 percent of propositions that start out behind get taken down.â€

Typically, ballot measures start out ahead, but become less popular as the opposition campaign begins raising questions and creating doubt, he said.