The Secretary shall conduct a referendum among producers, who during a representative period determined by the Secretary have been engaged in the production of potatoes, for the purpose of ascertaining whether the issuance of a plan is approved or favored by such producers. When the issuance of a plan would subject importers to the terms and conditions of a plan, the Secretary also shall conduct the referendum among importers, who during a representative period determined by the Secretary have been engaged in the importation of potatoes, for the purpose of ascertaining whether the issuance of such plan is approved or favored by such importers.