Organized labor's advertising offensive against Proposition 32 appears to be paying off.

According to a new statewide poll, 53% of likely voters say they support the November ballot measure that promises to eliminate special-interest money in politics, a drop of nearly 10 percentage points since the beginning of August.

The slipping support comes as the labor-backed opposition campaign airs statewide radio ads casting the initiative as "a deceptive proposition stuffed with special exemptions" for businesses and corporate-funded super PACs. The proposition's backers dispute the claim, saying the initiative uses language that mirrors federal law and bans all corporations from giving directly to candidates.

"It seems some of the messages in the advertisements are sticking with voters," said Dr. Michael Shires of Pepperdine University, which sponsored the new poll in partnership with the California Business Roundtable. "However, the 'yes' campaign is yet to launch their campaign, which should impact the measure's support."

Proposition 32 is labor's top priority this year.

Though the measure would prohibit unions and corporations from contributing directly to candidates, labor leaders are more concerned about another provision in the initiative: banning the practice of political contribution by payroll deduction, the primary method unions use to raise political cash.

So far, unions have raised more than $35.7 million to defeat the initiative, compared with about $3.1 million raised in support of the measure.

The new poll, conducted by M4 Strategies Sept. 9-12, was an online survey of 802 likely California voters. The margin of error was 3.5 percentage points.