Any such contract with the United States or any department or agency thereof, or with any private corporation organized under the laws of the United States, by which the district or an improvement district thereof incurs an indebtedness or liability exceeding in any year the income and revenue for such year, shall not be executed without the assent of a majority of the qualified electors of the district or the improvement district voting at an election to be held for that purpose. Such election shall be called and held, so far as practicable, in the same manner as bond elections for the district or an improvement district.