While Congress debates a nationwide standard for labeling genetically modified foods, Vermont residents may be wondering why they can't get their favorite soda.

On July 1, the state required most foods to be labeled if they have any genetically modified organisms in the ingredients. Since then, manufacturers have told retailers that more than 3,000 products won't be shipped to the state because they don't comply with the state law, the strictest in the nation.

The implementation of the law serves as a backdrop to a major effort in Congress to adopt nationwide GMO labeling laws that would pre-empt measures in Vermont and other states.

The Senate voted 63-30 Thursday on a bill — a compromise from Sens. Pat Roberts, R-Kan., and Debbie Stabenow, D-Mich. — that would require GMO labeling, but in different ways. For instance, a company can put a label on its product or use a QR code that a person can scan with his smartphone to get more information via a website.

The bill would supersede any existing state laws on GMO labeling, and some lawmakers believe the Senate had a particular state in mind: Vermont.

"Make no mistake: Vermont's first-in-the-nation [genetic engineering] labeling law is what is under attack here," said Sen. Patrick Leahy, D-Vt., on the Senate floor Thursday. "Vermonters' carefully debated law is the catalyst that has driven millions of lobbying dollars to the doors of the United States Senate."

In the week since the law went into effect, grocers say state residents had better get used to some holes in store shelves.

Manufacturers have told grocers that about 3,500 products will not be relabeled to comply with the law, according to a representative with Price Chopper, a nationwide grocery chain with 15 Vermont stores.

The 3,500 products are out of 35,000 that are authorized in the stores, said Mona Golub, a spokeswoman for the 135-store chain.

The law does exempt certain foods such as cheeses, meats and alcoholic beverages.

"We will reset our shelves to make sure that we have all products that are in compliance, of course, and if there are other products that can fill those holes that are compliant then we will offer them," Golub said.

The products range from flavored ketchup to even jarred herrings.

The grocery chain decided about a year ago to re-label the 9,000 store-brand products it offers in every store, not just those in Vermont.

Golub said she didn't have an estimate on how much that cost, but noted that it took hundreds of hours of labor to research which ingredients had GMOs.

Price Chopper employees also had to call suppliers to find out which outside products were complying with the law.

Golub said the extra work could result in higher prices for shoppers, but declined to specify how much.

"When the cost of goods go up and in this case relabeling is factored into it, it will be partially reflected in the retail of those goods," she said.

Some major food suppliers have chosen to comply with the law and re-label all of their products nationwide, including General Mills, Kellogg and Mars. Others are being more selective and scaling back some of their products sent to Vermont.

Take Coca-Cola, which has fought the Vermont law and is lobbying Congress to pass a national GMO standard.

The soda giant told the Washington Examiner that it might make some products temporarily unavailable to the state to avoid "multiple labeling changes."

So you can still get Coke, Diet Coke and Coke Zero in Vermont, but maybe not Fanta Mango.

Vermont lawmakers say that they aren't worried that prices for certain products could rise as a result of the law.

Leahy told a local TV news crew that Coke sales have been "going down."

"They got something that will help the sales go back up," he said, referring to the law. Leahy was among the senators who voted against the Senate GMO compromise that passed 63-30 earlier this week.

Vermont Sen. Bernie Sanders also opposed the bill, noting that it was not as strict as Vermont's provision.

The Senate bill "contains huge loopholes and imposes no federal penalties whatsoever for violating the labeling requirement," Sanders said. "If it becomes law, the legislation will pre-empt Vermont's standard and create chaos for other states that have passed similar bills."

Maine and Rhode Island have passed GMO labeling laws, and more than 10 states have proposed such laws, according to the Center for Food Safety, a nonprofit that advocates for GMO labeling.

Debate on the bill was momentarily sidelined earlier this week when a protester with the group Organic Consumers Association dropped $2,000 in cash on the Senate floor to protest influence over the GMO vote.

The bill's fate is not clear in the House, which earlier this year passed laws that do not allow any state to adopt a GMO labeling law.