California Gov. Jerry Brown (D) has vetoed a measure that would have severely limited the ability of wealthy activists and corporations to use paid signature gatherers to get initiatives on the ballot.

The measure, Assembly Bill 857, would have required 10 percent of signatures for any given ballot initiative to be collected by volunteers, rather than by paid signature gatherers. The number of signatures supporters need to turn in is based on the number of votes in the last gubernatorial election; that means groups would have to rely on volunteers to gather a little more than 50,000 of the 504,760 valid signatures required to get an initiative on the ballot.

“The initiative process is far from perfect and monied interests have historically manipulated it at will,” Brown wrote [pdf] in a veto message. “Requiring a specific threshold of signatures to be gathered by volunteers will not stop abuses by narrow special interests — particularly if ‘volunteer’ is defined with broad exemptions as in this bill.”

Those exemptions included employees or members of nonprofit organizations, who could be considered volunteers. That provision, the bill’s opponents say, allows unions to count signatures collected by their members as volunteer-collected, giving labor organizations an advantage in qualifying ballot measures.

As the number of required signatures has grown, so has reliance on paid signature gatherers. Deep-pocketed interests, whether business groups or wealthy individuals that fund the signature drives, frequently pay $5 or more for every signature a gatherer turns in. And because many signatures turn out to be invalid, strategists backing a given initiative routinely strive to turn in hundreds of thousands of extra signatures; simply paying the signature gatherers can cost $3 million to $4 million, or more, even before a traditional campaign begins.

Changing the initiative rules would have a disproportionate impact on Republicans and conservative groups. With Democrats dominating the state legislature, the initiative process is almost the only way conservatives can affect policy in California. Requiring so many signatures to be collected by volunteers would put a much bigger onus on those conservative groups.

“Democrats hold a huge majority in both houses [of the California legislature] and have been able to legislate whatever they want with impunity. The only outlet allowed for voters to even tap the brakes on an agenda they may not support is to utilize the initiative and referendum process,” said Michael Arno, a longtime political consultant who specializes in initiatives. The bill, Arno said, would have “rid the people of a legitimate right to make their own laws.”

Brown said efforts to reform the initiative process were worth considering. Democrats are likely to try again to reform the system in the next legislative session; the measure was initially sponsored by the California Labor Federation.

The veto is the second time in two days that Brown has rejected a major Democratic priority passed by the state legislature. On Friday, Brown vetoed several measures that would have imposed new restrictions on semi-automatic weapons.