Organized labor has begun writing big checks in a move that threatens to defeat a key budget measure in the May 19 special election.

The Service Employees International Union State Council contributed $500,000 this week to defeat Proposition 1A, a spending limit and temporary tax hike that has drawn diverse opposition but little funding until now.

Gov. Arnold Schwarzenegger and legislative leaders took a calculated risk in February when they crafted a budget package that relies on cooperation from frustrated voters. They figured they could overcome that hurdle with millions of campaign dollars – as long as no serious opposition emerged.

"It just got a lot harder," said Dan Schnur, director of the University of Southern California's Jesse M. Unruh Institute of Politics and a former Republican strategist.

"The biggest advantage the proponents have had all along is the lack of a well-funded opposition," Schnur said. "Historically, you don't need to outspend ballot measures to beat them, and in a low-turnout election this is a decent amount of money."

Proposition 1A has become the lightning rod of the six measures because it includes pieces that enrage groups on both the political right and left.

The proposal attempts to limit spending in good fiscal years, building a "rainy-day" fund the state could tap in bad times. Public employee unions such as SEIU fear that eventually would mean reduced spending on health care and education as well as fewer jobs for their members.

Anti-tax groups oppose Proposition 1A because it contains roughly $16 billion in temporary taxes between 2011 and 2013. Democrats asked that the taxes be included, and leaders believed the proceeds would be sufficient to keep the unions quiet.

Until last week, the unions opposed Proposition 1A in silence. But SEIU, the California Federation of Teachers and the California Faculty Association formed a committee that has raised $1 million.

"We have said from the outset that we are going to be running an aggressive and spirited campaign to reach out to voters and educate them about the serious flaws in Proposition 1A and the consequences for California's future," said Mike Roth, an SEIU spokesman.

Proponents still have the benefit of fundraising titans at their disposal. The California Teachers Association is on the same side as the California Chamber of Commerce and Schwarzenegger, even though the teachers union had considered them fierce foes in the 2005 special election.

CTA has raised $6 million on its own to spend on statewide television ads, including $1 million from the National Education Association. Schwarzenegger has helped a separate committee, Budget Reform Now, raise an additional $6 million. CTA is motivated by Proposition 1B, which would provide $9.3 billion to schools as long as Proposition 1A passes as well.

Proponents are also challenged by the confusing nature of this election. Many of the six ballot measures deal with complex budget formulas. Voters cannot rely as easily on endorsements, because groups representing similar causes are divided.

The Congress of California Seniors opposes Proposition 1A, yet AARP California endorsed the measure Tuesday. The California Federation of Teachers is on the opposite side of CTA. The business-backed California Taxpayers Association supports the measure, while the Howard Jarvis Taxpayers Association opposes it.

"The challenge for the 'yes' side is how to simplify this in a way that makes people feel comfortable this is going to do something," said Bill Carrick, a Democratic strategist.

The Budget Reform Now campaign is asking voters to approve all six ballot measures in tandem.

"Propositions 1A through 1F are together a package of reforms that will bring in much-needed revenues in the short term but also tackle the state's long-term budget issues," said campaign spokeswoman Julie Soderlund.

That strategy has the added effect of tying each ballot measure to the state's current budget problem, even though Proposition 1A has a long-term impact and would not directly alter the budget until 2011.

"Proponents have to convince the angry voters that this is part of the solution to solve what they are mad about," said Tony Quinn, co-editor of the California Target Book, a nonpartisan analysis of legislative and congressional elections.

But tying all the measures together could have a negative impact as well. The California Republican Party this weekend voted to oppose all six measures in part because proponents had bundled them together, according to Mike Spence, a party official. Voters may go against all of them based on their strong feelings about one particular measure, which is why opposition to Proposition 1A could prove significant.

"Right now there's a tremendous tendency to reject anything out of Sacramento," said Republican strategist Dave Gilliard.

Call Kevin Yamamura, Bee Capitol Bureau, (916) 326-5548.