FRANKFURT — The chlorinated chickens are back. That’s a bad sign for free trade.

The birds are being used in a food fight by populists in Europe who are poised to make significant gains in parliamentary elections this week. Such a shift in the makeup of the European Parliament would complicate the simmering trade conflict between the United States and Europe.

Chicken meat from the United States is routinely sterilized using a chlorinated wash, a method forbidden in the European Union. The American birds are banned and often cited, with some dread, as an all-purpose justification for putting up barriers to American products.

The Trump administration has tried to put the chickens back on the negotiating table, notably with Britain as it prepares to leave the European Union. European candidates, both on the left and on the right, have seized the cluckers as a way to dramatize the stakes in the balloting that runs from Thursday to Sunday.

Among the first candidates to bring up chickens was Yanis Varoufakis, a left-wing former finance minister of Greece who is running for a seat in Brussels. During an appearance in March, Mr. Varoufakis vowed to block multinational corporations that “will want to introduce chlorinated chickens in Europe.”