WASHINGTON — The Senate will grapple this week with perhaps the most contentious issue in the food industry: whether the government should require mandatory labeling on foods containing genetically engineered ingredients.

On Wednesday, the Senate is set to vote on a measure that would create voluntary national standards for labeling food with genetically modified ingredients. The bill would prevent states from mandating labels just before Vermont was set to become the first in the nation to impose such requirements.

But the measure most likely lacks sufficient support from Democrats, most of whom would like to see a mandatory labeling program that offers food manufacturers different options for presenting the information, including a simple symbol. That means the legislation will almost certainly have to be revised.

“Voluntary standards are no standards at all,” said Senator Jon Tester, Democrat of Montana and an organic farmer. “We need to defeat this bill,” he added. “This is bad, bad, bad policy.”