Indian tribes, horseracing interests and a labor union are stepping up a battle over casino referendums on the Feb. 5 ballot, adding $20 million to the jackpot that advertisers and campaign consultants could eventually collect.

Four tribes are attempting to sell voters on agreements that would allow them to add as many as 17,000 slot machines to the 8,000 already at their casinos. In December alone, the tribes poured $15.6 million into a joint campaign backing the four referendums, bringing their total spending to $44.5 million, according to the secretary of state's office.

One of the referendums, Proposition 94, would allow the Pechanga Band of Luiseno Indians to add 5,500 slots to the 2,000 now at its casino just south of Temecula. Pechanga has put $20 million into the joint campaign since summer, including a $10 million contribution Dec. 21, and also appears to have stepped up advertising on at least one local television station.

Under an agreement, or "compact," signed by Pechanga and Gov. Arnold Schwarzenegger in August 2006, Pechanga would pay the state 15 percent of its net winnings on the first 3,000 slots it adds and 25 percent of the net winnings on the next 2,500 machines. The compact calls for a minimum payment of $42.5 million each year, up from the $29 million minimum under its current arrangement. The compact would expire in 2030.

The other three compacts allow similar terms for the Sycuan Band of the Kumeyaay Nation's casino near El Cajon, the Morongo Band of Mission Indians' casino near Banning and the Agua Caliente Band of Cahuilla Indians' casinos in Palm Springs and Rancho Mirage.

The Legislature ratified all four compacts in June over protests from a hotel workers' union and the owners of Bay Area racetracks. The opponents responded by gathering signatures from a half-million voters and placing the issue on the February ballot.

A survey the non-partisan Field Poll conducted in October showed 52 percent of voters favoring the expansions and 35 percent opposed, a margin that the independent pollster called surmountable. A smaller-scale poll that Field released last week showed voters in favor by a margin of just 39 percent to 33 percent.

The companies that own television stations see a string of sevens. The tribes' campaign reported spending more than $7 million on advertising and more than $1 million on mailers in late September.

"With a Feb. 5 primary, we have an abbreviated window to get our message across to voters," said Roger Salazar, a spokesman for the "Yes on 94, 95, 96 & 97" campaign.

The campaign has focused its resources on television advertising, in contrast to the lower-budget campaign run by opponents. From the $4.6 million the "no" campaign raised through Nov. 30, it spent $3 million on the petition drive. In an interview last month, spokesman Al Lundeen said the campaign would continue to focus on reaching voters directly and through news media in the short run, but refused to rule out a big-budget media campaign.

Since then, his campaign has raised another $4.5 million, including nearly $1.8 million from other Indian tribes, including the Pala Band of Mission Indians, which operates a 2,000-slot casino just seven miles south of Pechanga's. Unite-HERE, a hotel workers' union that has sought to unionize employees at some of the casinos, kicked in $2 million in mid-December. The anti-expansion tribes have spent nearly $1 million in a separate but parallel effort, mostly to support the petition gathering.

The "Yes" campaign's 30-second advertising spots have been running in all of California's sizeable media markets, from San Diego to San Francisco and Sacramento. None specifically mention the referendums on the February ballot, though one of the five urges viewers to "support the Indian gaming revenue agreements."

In another, San Miguel's fire chief touts the revenue the casinos will bring the state. Schwarzenegger and the four tribes say they could add up to $9 billion over the lives of the compacts, though the Legislature's nonpartisan analyst has cautioned that the figure is probably overstated. Several of the ads mention the state's current budget deficit.

In yet another, the camera pans across a springtime bloom of wildflowers in a desert. That ad urges viewers to consider the compacts' potential effects on funding for public safety and schools.

Such ads probably won't sway voters who oppose gambling on moral grounds or because they fear increased crime or heavier traffic in their neighborhoods, said Tom Hollihan, a professor at USC's Annenberg School of Communications who studies political campaigns.

But they could have an impact on the large numbers of voters who are undecided on or unfamiliar with the issue, he said. The Field poll released last week found that 28 percent of voters were "undecided" on the expansions, and that 73 percent hadn't previously heard of the four referenda.

"The casinos probably can't convert the true believers," Hollihan said. "But they probably can respond by creating messages that appeal to the mainstream. You have to get these people through advertising that's ubiquitous."

Contact staff writer Chris Bagley at (951) 676-4315, Ext. 2615, or cbagley@californian.com.