SAN FRANCISCO—California's largest gay rights group on Monday accused the backers of a ballot measure seeking to repeal a law requiring gay history to be taught in public schools of deliberately hiding the size and source of campaign contributions.

Two conservative groups behind the Stop SB 48 campaign "may have engaged in an unlawful scheme" to violate campaign reporting rules, Equality California Executive Director Roland Palencia said in a complaint filed with the state Fair Political Practices Commission.

Palencia's group accuses Capitol Resource Institute and Pacific Justice Institute, the organizations that have taken the lead on undoing the first-of-its kind law, of raising and spending money to qualify the repeal referendum for the June 2012 ballot without registering as campaign committees.

Under California's strict campaign finance laws, political entities that receive more than $1,000 in contributions are required to register with the secretary of state, said Cary Davidson, an election law lawyer on the Equality California board....