In California's ever-expensive ballot wars, voters typically know who funds advertisements that hold great sway with the electorate.

But that may be changing.

An Arizona-based nonprofit named Americans for Responsible Leadership gave $11 million this week to defeat Gov. Jerry Brown's tax initiative and curb unions' political power without saying where a single dime originated.

Campaign watchdogs say such donations exploit a loophole in California campaign finance law. Nonprofits can shield donors as long as money was never earmarked, with the burden of proof falling on state regulators already deluged with work.

"It is vitally important information for the public," said Jessica Levinson, a professor at Loyola Law School who formerly worked at the watchdog group Center for Governmental Studies. "The identity of a speaker allows the public to evaluate the credibility of the claims that are made by using this money."

Political consultants said Wednesday the $11 million donation is probably the largest sum given in a statewide contest without a known source. Last month, Iowa-based American Future Fund gave $4 million for ads backing Proposition 32 without disclosing its donors.

During election season, California requires campaigns to report large donations within 24 hours. In other instances this fall, wealthy individuals such as Republican activist Charles Munger Jr., attorney Molly Munger and hedge-fund manager Tom Steyer were named as they wrote multimillion-dollar checks.

But donors can stay anonymous by giving to a nonprofit, which has to disclose only if the contributors specified where the money must go, said Gary Winuk, enforcement chief at the California Fair Political Practices Commission, the state's watchdog arm.

To bring a case forward, regulators must prove that donors earmarked their dollars.

"Short of checking every transaction that happens, it's logistically next to impossible," Winuk said. "You have to have somebody on the inside."

Staying anonymous has its advantages. Businesses may fear scaring off customers or becoming targets in a campaign, as oil and tobacco donors have in recent years.

Individuals may want to avoid personal consequences. In 2008, for instance, donors to same-sex marriage ban Proposition 8 received angry emails and phone calls.

"These donors are very wealthy people who are publicity-shy and have business interests," said Democratic strategist Steven Maviglio. "They don't want the spotlight on themselves because it could backfire."

Maviglio worked on the 2010 campaign opposing Proposition 23, which attempted to reverse the state's tight greenhouse-gas restrictions. The Missouri-based Adam Smith Foundation gave $498,000 toward the measure. The group was organized by Missouri conservatives who refused to say where funds came from.

"These anonymous donations to campaigns are increasingly a headache because it's usually well coordinated behind the scenes but never really sees the light of day," Maviglio said. "Anybody who says differently is lying through their teeth."

The U.S. Chamber of Commerce spent $431,000 on television ads in late summer criticizing California government, a message that may have worked against Brown's tax initiative. The group said it does not "publicly discuss the funding of our programs."

Conservative activist Ward Connerly settled with the state FPPC after he refused to name donors to his nonprofit American Civil Rights Coalition, which funded a 2003 initiative to ban government collection of racial data. The FPPC accused Connerly of illegally routing funds through his nonprofit to avoid disclosure. As part of the 2005 settlement, he revealed donors including San Diego Padres owner John Moores and media mogul Rupert Murdoch.

While such examples involve conservative organizations, Republican strategist Rob Stutzman pointed to environmental nonprofits that backed Proposition 21 of 2010, which would have raised the vehicle license fee $18 annually to fund state parks.

"They're all operating under the same law," Stutzman said. "You ultimately don't know who the donors are."

Stutzman said treating established foundations differently "becomes a value judgment that an organization is OK. You either have one set of rules or not."

Left-leaning Consumer Watchdog has drawn criticism for cloaking donors while engaging in political activity, such as opposing Proposition 33 this year allowing insurers to offer discounts based on length of coverage. "They're a classic example of an organization carrying the water in all likelihood for trial lawyers but who never disclose their contributors," Stutzman said.

The group's president, Jamie Court, said Consumer Watchdog does not fund its political activity with donations and relies instead on legal awards from successful cases against insurers.

Americans for Responsible Leadership gave its eight-figure check to Small Business Action Committee PAC on Monday, according to a bare-bones filing. The PAC opposes Brown's Proposition 30 and supports Proposition 32, a measure labor unions consider so serious a threat that they have pooled $53 million from member dues to defeat it. The coalition includes a who's who of California unions representing teachers to warehouse employees.

Beth Miller, spokeswoman for the small-business PAC, said she did not know the identity of the individual donors to Americans for Responsible Leadership.

The organization formed last July as a nonprofit, according to an Arizona filing. Its three directors are businessmen active in GOP circles.

Robert Graham operates his own wealth-management company and has run for governor and state Republican Party chairman. In one online video, he describes labor unions as "parasitic" like mistletoe.

Another director, Eric Wnuck, unsuccessfully ran for Congress in 2010 as a Republican. His Twitter feed from 2009 criticizes the federal health care overhaul and says "illegal immigration is crippling Arizona" but also promotes renewable energy.

The group's directors could not be reached Wednesday.

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