Sacramento --

A socially conservative organization based in Sacramento filed documents Friday afternoon to start a voter referendum on a controversial law that adds the contributions of lesbian, gay, bisexual and transgender Americans to school instruction.

The proponent of the proposed referendum, Paulo Sibaja, filed a request for a title and summary with the attorney general's office. Sibaja said he acted on behalf of the Capitol Resource Institute, which had officially opposed the bill throughout the legislative process before Gov. Jerry Brown signed it Thursday. Sibaja is the legislative director of that organization.

The Capitol Resource Institute is a hard-line, socially conservative organization that has long opposed efforts in California to expand rights for the LGBT population. Backers eventually would have to collect 504,760 signatures to allow voters to decide whether to keep the law in place or reject it.

Sibaja said that a coalition has formed behind the proposed measure, though he would not name the other members. He said a news conference Wednesday would give more details. When asked how the group planned to fund the referendum, he said, "That's what the press conference will be about."

The law - the first such in the nation - requires that, beginning Jan. 1, public schools in California must teach students about the contributions of lesbian, gay, bisexual and transgender Americans as part of the social sciences curriculum. Textbooks will also include information on the role of LGBT Americans, as well as Americans with disabilities, though the state budget crisis has delayed the purchasing of new books until at least 2015.

Sen. Mark Leno, D-San Francisco, who introduced the Senate Bill, said he was not surprised that the institute, which he called "extremist," took this step.

"I think it will be a challenge for them to get the signatures," Leno said.

"If they succeed in that, I bet Californians" reject it, he added.

Referendums are similar to voter initiatives, but are not as common. They allow for voters to directly decide on bills that are passed by the Legislature, though budget bills are excluded. The online retail giant Amazon this week also filed with the attorney general to begin a referendum on a tax measure that was part of the budget.

Democratic lawmakers have called the Amazon move unconstitutional because the law was a budget bill.

This article has been corrected since it appeared in print editions.