Any political party desiring to permit registered voters who are not affiliated with a political party to vote for candidates of that party in the primary election shall file a letter stating that the governing body of the political party has adopted a rule allowing registered voters who are not affiliated with a political party to vote in the primary election for candidates of that party. The letter and copy of the adopted rule shall be filed with the Secretary of State at least sixty days before the primary election. The Secretary of State shall notify the appropriate election commissioners and county clerks in writing that the political party filing the letter will allow registered voters who are not affiliated with a political party to vote in the primary election for candidates of that party. Once filed, the rule allowing such voters to vote in such primary election shall be irrevocable and shall apply only to the primary election immediately following the adoption of the rule.