Upon petition of electors in any city equal in number to twenty-five percent of the votes cast for all candidates for mayor at the last preceding city election therein, the mayor by proclamation shall cause to be submitted the question of organizing the city under the commission form of government at a special election at a time specified therein and within sixty days after the filing of the petition. If the plan is not adopted at the special election called, it shall not be resubmitted to the voters of the city for adoption within two years thereafter.