LOS ANGELES — Advocates for the labeling of genetically modified food vowed to carry their fight to other states and to the federal government after suffering a defeat in California on Tuesday.

A ballot measure that would have made California the first state in the nation to require such labeling was defeated, 53.1 percent to 46.9 percent. Support for the initiative, which polls said once was greater than 60 percent, crumbled over the last month under a barrage of negative advertisements paid for by food and biotechnology companies.

The backers of the measure, known as Proposition 37, said on Wednesday that they were encouraged it had garnered 4.3 million votes, even though they were outspent about five-to-one by opponents. They are now gathering signatures to place a similar measure on the ballot in Washington State next year.

Declaring that more than four million Californians are “on record believing we have a right to know what is in our food,” Dave Murphy, co-chairman of the Proposition 37 campaign and executive director of Food Democracy Now!, an advocacy group, said on Wednesday: “We fundamentally believe this is a dynamic moment for the food movement and we’re going forward.”

Still, there is no doubt the defeat in California has robbed the movement of some momentum. Until Tuesday’s vote, labeling proponents had been saying that a victory in California, not a defeat, would spur action in other states and at the federal level.

The defeat greatly reduces the chances that labels will be required, according to L. Val Giddings, a senior fellow at the Information Technology and Innovation Foundation, a Washington organization supporting policies that favor innovation. “I see little potential that the defeat in California could result in any increase in pressure for labels. ”

Dr. Giddings, who is a supporter of biotech crops, said it would now be more difficult for labeling proponents to raise money. “What justification can they present to their funders to pour more money down this drain?” he said.

The election in California was closely watched because it had national implications. It could have led to a reduction in the use of genetically modified crops, which account for more than 80 percent of the corn, soybeans and sugar beets grown in the United States. That is because food companies, fearing that some consumers would shun products labeled genetically engineered, would instead reformulate their products to avoid such ingredients.

With so much at stake, food and biotechnology companies amassed $46 million to defeat the measure, according to MapLight, an organization that tracks campaign contributions. Monsanto, the largest supplier of genetically engineered seeds, contributed $8.1 million. Kraft Foods, PepsiCo and Coca-Cola each contributed at least $1.7 million.

The backers of Proposition 37 raised only $9.2 million, mainly from the organic and natural foods business.

The proponents argued that people have a right to know what is in their food. They said that genetically engineered crops have not been adequately tested and that dozens of countries require labeling.

The Food and Drug Administration does not require labeling of a food just because it is genetically modified, saying there is no material difference between such foods and their conventional counterparts.

The big food and biotechnology companies argued that numerous expert reviews have shown the crops to be safe. For the most part, they did not directly attack the notion of consumers’ right to know. Rather they said Proposition 37 was worded in a way that would lead to red tape, increases in food prices and numerous lawsuits against food companies and supermarkets.

Some backers of labeling will shift their focus to Washington, hoping to get the F.D.A. to change its mind and require labeling.

“We think that attention is now going to shift back to Washington, with a whole lot more to discuss and a whole lot more people interested,” said Gary Hirshberg, the chairman of Stonyfield, an organic yogurt company.

Mr. Hirshberg is also chairman of Just Label It, a group that submitted a petition with more than one million signatures to the F.D.A. asking it to require labeling. So far, however, the F.D.A. has shown little propensity to overturn its policy. And bills in Congress to require labeling have failed to gain much support.

Proposition 37 has no doubt raised awareness, however, which might prompt some consumers to seek foods that do not contain genetically engineered ingredients.

“Everything you buy in the grocery is a vote,” said Sara Hadden of Hermosa Beach, who organized street-corner rallies in favor of Proposition 37. “That’s the vote that really counts.”

One question is whether food firms, having narrowly escaped a disruption of their business on Tuesday, will make changes on their own — like voluntarily labeling or reducing their use of genetically modified crops.

If that is being considered, the food companies are not letting on. In a statement Wednesday, the Grocery Manufacturers Association, which represents big food companies, called the defeat of Proposition 37 “a big win for California consumers, taxpayers, businesses and farmers.”