Since 1968, the Secret Service has been tasked to provide federally-funded security protection to presidential candidates. The expense, according to the Secret Service's website, "is designed to maintain the integrity of the democratic process and continuity of government."
The authority for such protection is found at 18 U.S.C. 3056(a)(7). The Statute provides for Secret Service protection for those candidates the Secretary of Homeland Security identifies after consultation with various named public leaders. It is not mandatory or automatic.
Tax money must cease being used on a candidate that has (a) brought the federal judiciary into disrepute based on race and (b) has insinuated "the Second Amendment" (gun ownership) should be used as a tool against his political opponents.