The candidates for electors of president and vice president of any political party or group of petitioners shall not be placed on the ballot, but in the years in which they are to be elected the names of candidates for president and vice president, respectively, of such parties or group of petitioners shall be placed on the ballot, as the names of candidates for United States senators are placed thereon, under their respective party, petition, or adopted titles for each political party, or group of petitioners, nominating a set of candidates for electors.