The Secretary shall conduct a referendum as soon as practicable among end product manufacturers not exempt hereunder who, during a representative period preceding the date of the referendum, as determined by the Secretary, have been engaged in the manufacture of end products, for the purpose of ascertaining whether the issuance of an order is approved or favored by such manufacturers. Qualified end product manufacturers may register with the Secretary by mail to vote in such referendum during a period ending not less than thirty days prior to the date of the referendum. Within ten days thereafter, the Secretary shall determine which end product manufacturers are eligible to vote in such referendum and cause to be published the list of such eligible voters. The Secretary shall issue ballots to all such persons who have so registered and been declared eligible to vote. No order issued pursuant to this chapter shall be effective unless the Secretary determines (1) that votes were cast by at least 50 per centum of such registered end product manufacturers, and (2) that the issuance of such order is approved or favored by not less than two-thirds of the end product manufacturers voting in such referendum or by a majority of the end product manufacturers voting in such referendum if such majority manufactured end products containing not less than two-thirds of the total processed wheat contained in all end products manufactured by those voting in the referendum, during t