With less than a week before voters in two U.S. states weigh measures to require labeling of foods made with genetically modified ingredients, labeling supporters were hoping for victory but planning for defeat.

Fresh polling showed support for the Oregon GMO labeling law waning in the face of a well-funded onslaught of advertising from labeling opponents, and pro-labeling Colorado campaigners still could not muster enough money for television ads to counter thousands of ads by labeling opponents.

In both states, the bid for votes on Nov. 4 is largely coming down to the size of each side's bank account. Those opposing mandatory GMO labeling have contributed more than three times the money contributed by labeling supporters.

"We're not able to compete with these massive contributions,' said Larry Cooper, campaign chair for Right to Know Colorado, which is asking backers to go door to door to drum up support for labeling. "I have not written off the campaign. But it is very much a David and Goliath situation."

The Colorado campaign counts about $900,000 in contributions, mainly from organic groups and scores of individuals, many making $10 or $20 donations. That compares to more than $16 million contributed by food companies and seed and chemical firms for opposition.

In Oregon, labeling supporters have received more than $7 million, compared to the $18.7 million put together by opponents.

The money has fueled heavy advertising. The Oregon and Colorado measures are among the most heavily advertised measures on the Nov. 4 ballot nationwide, with more than 9,600 ads focused on GMO labeling in the two states, according to the Center for Public Integrity.

An Oct. 26-27 poll of 403 likely voters in Oregon showed the labeling measure losing 48 to 42, according to a survey conducted by Elway Research of Seattle. The poll had a margin of error of five percentage points, plus or minus.

David Bronner, whose California-based natural products company Dr. Bronner's Magic Soaps has contributed $2 million to the Oregon labeling campaign and $50,000 to the Colorado campaign, said large donors like himself pegged Oregon as the best chance to pass a labeling law this year.

"I would love it if our money had enough firepower to run two campaigns, but we don't. So we're focusing on Oregon," he said. Even if both measures lose, labeling efforts will continue, Bronner said.

"We aren't going away," he said.

One opponent of GMO labeling is biotech crop company DuPont Pioneer, a unit of DuPont, which on Oct. 17 kicked in $4.5 million to the anti-labeling campaign in Oregon.

"Dupont is part of a broad coalition of people in Oregon that oppose this labeling initiative because it could be costly and confusing for consumers," said DuPont spokeswoman Jane Slusark.

Several states are eying GMO labeling measures. Labeling proponents say GMOs can be harmful for humans and the environment, while labeling opponents say GMOs are proven safe.

(Reporting By Carey Gillam in Kansas City, Mo.; Editing by Meredith Mazzilli)