The tribe that owns Thunder Valley Casino near Rocklin is the largest campaign contributor in a high-priced effort to overturn slot machine compacts the governor negotiated with four big Southern California casinos. The United Auburn Indian Community has donated $5 million in the fight against Propositions 94, 95, 96 and 97, which if approved would allow the four Southern California tribes to add 17,000 slot machines – more than double the 8,000 they operate today. The United Auburn tribe’s casino has 2,700 slot machines as part of a 25-year compact reached with the state in 2004. Tribal leaders are worried that the outcome of the propositions could help its soon-to-be rival – the Shingle Springs Band of Miwok and that tribe’s Foothill Oaks Casino off Highway 50 near Placerville, scheduled to open this fall.