Haselmayer was an early adherent of the Nazi Party, joining it on 1 April 1925 (membership number 36) shortly after the lifting of the national ban that had been imposed on it in the wake of the failed Beer Hall Putsch.
Like all Gauleiters, Haselmayer was directly responsible to Hitler and was his personal representative to the Gau.
On 28 July, Hitler wrote a letter to Haselmayer wishing him a speedy recovery and hoping that he would soon be well enough to exact revenge on the perpetrators.
In March 1936 he obtained a law license and began working as an attorney in Munich.
On 5 February 1937, his final petition for clemency and for readmission to the Party was denied.