When a candidate of any political party has filed a nominating petition or filing fee for state senator or representative and has been nominated for the office by a party, he or she shall not be permitted to withdraw unless he or she shall be certified as a nominee at the subsequent state convention of the same party for a statewide office, or has removed from the district, or has become physically unfit, or become disqualified for any reason. If certified by a state convention for a statewide office, the candidate shall be deemed to have withdrawn from the previous nomination. No such vacancy shall be filled by the county executive committee or committees except for the causes and as herein specified. This prohibition shall not be construed to prohibit the withdrawal of any candidate who has been nominated without having filed a nominating petition or filing fee and whose name has been written or placed on the ballot of any political party.