Two tribes in Placer and San Diego counties announced Thursday that they will donate up to $1 million toward a petition drive for a state referendum seeking to bar four Southern California tribes from adding 17,000 new slot machines.

Attorney Howard Dickstein said the United Auburn Indian Community and the Pala Band of Mission Indians will contribute $500,000 each to a campaign committee -- California Indian Tribes for Fair Play -- that will support the signature gathering.

United Auburn operates the Thunder Valley casino near Sacramento -- one of the state's most successful gambling facilities. Pala runs the Pala Casino Spa & Resort in northern San Diego County, a competitor to the tribes seeking casino expansions.

Their announcement increases the likelihood of a bruising ballot showdown over controversial gambling agreements that Gov. Arnold Schwarzenegger signed and the Legislature approved with the Morongo Band of Mission Indians, the Pechanga Band of Luiseño Indians, the Agua Caliente Band of Cahuilla Indians and the Sycuan Band of the Kumeyaay Nation.

Representatives for a hotel and casino workers union -- UNITE HERE -- and the Hollywood Park and Bay Meadows horse racing tracks announced a petition drive July 27 to force a statewide vote on gambling compacts for the tribes. The agreements would allow the wealthy tribes to add up to 3,000 to 5,500 new slot machines each -- potentially dwarfing the largest casinos in Las Vegas.

Now Dickstein said the United Auburn and Pala tribes are joining the fight -- at least to get the referendum qualified for the Feb. 5 state ballot. He said they believe side agreements on casino oversight issues that were negotiated to facilitate passage of the compacts in the Legislature circumvent federal law.

"What they (United Auburn and Pala) have decided is to provide financial support for the signature gathering," Dickstein said. "If and when it qualifies for the ballot, they will have a fresh look at what, if any, participation they will have in the campaign."

Opponents of the Southern California gambling compacts must submit 433,971 valid signatures of registered voters by Oct. 8 for four separate referendums to qualify the measures for the Feb. 5 ballot. If the measures qualify, they seem sure to trigger a very expensive ballot fight.

"We will vigorously defend these compacts that will bring in new revenue to California," said Patrick Dorinson, spokesman for the Morongo tribe -- which spent $20 million in an advertising and politicking blitz to get its gambling pact approved.

Pechanga tribal Chairman Mark Macarro said in a statement that he was "disappointed that fellow tribes would oppose these agreements." He said Pechanga had "respected" state agreements in 2004 that gave Pala, United Auburn and other tribes rights to add "unlimited slot machines."

But Pala and United Auburn object to the new gambling deals because the Southern California tribes stand to pay lower revenue-sharing payments to the state per new slot machine added, Dickstein said.

He added that the "huge expansion" of slot machines in Southern California "could have catastrophically adverse impacts" on poorer tribes operating casinos in the same market.

Schwarzenegger, in signing compacts with five Southern California tribes last year, said the deals could generate $13.4 billion to $22.4 billion in revenue-sharing payments to the state over 25 years. One of the tribes -- the San Manuel Band of Mission Indians in San Bernardino County -- is still waiting approval in the Legislature to add up to 5,500 new slot machines.