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SACRAMENTO – An advertising blitz against Proposition 37 has slashed support for the genetically engineered food labeling initiative on next month’s ballot and may endanger its prospects for voter approval, a new poll shows.

Proposition 37, which once was ahead statewide by more than a 2-1 margin, still leads 48.3% to 40.2% in the poll released Thursday by the Pepperdine University School of Public Policy and the California Business Roundtable. Undecided voters accounted for 11.5%.

But with 3 1/2 weeks left to go before the Nov. 6 election, pollsters now foresee a tightening race, as opponents continued a media blitz financed by $35 million in campaign contributions.

“This is a great example of the power of advertising,” said pollster Chris Condon of M4 Strategies, which conducted the online survey of 830 likely voters from Sunday to Wednesday. Participants were asked to read the official ballot description and pro and con arguments before responding.

The new poll results marked a big change from previous polls that showed broad support for the measure, with some showing backing in the high 60% range. The poll took place just as a pair of televised anti-Proposition 37 spots hit the air.

“A lot of money has been poured into the No side, and the effect has been dramatic,” Condon said. “It’s down 19 points” since the previous biweekly poll.

Both sides of the Proposition 37 battle reacted predictably to the latest sounding.

The Yes campaign has been hurt by “10 days of incessant, pounding lies” on television, said spokeswoman Stacy Malkan, “but in the end, Californians will value knowing what’s in their food, and we’re confident they will vote yes on Proposition 37.”

At the No on 37 campaign, spokeswoman Kathy Fairbanks said greater skepticism shown in the poll was the result of a combination of television advertising, news stories and newspaper editorials opposed to the measure. “The more people learn about Proposition 37, the less they like it,” she said.

What people don’t like, Fairbanks said, are the many exemptions in the initiative.