An election shall be held to authorize such assessment and may be called by the board of directors of the district in its discretion. Such an election shall be called upon presentation to the board of directors of a petition requesting the levy of the assessment and specifying the object and purposes for which the proceeds thereof shall be expended. Such petition must be signed by electors entitled to cast a number of votes at district elections equal to at least 15 per cent vote of the number of votes cast at the last district election.