What is the history of candidate/nominee protection?
Candidate and nominee protection was expanded to include major candidates for President and Vice President in 1968:
Major candidates and their spouses began receiving protection after the assassination of Robert Kennedy in 1968. PL-90-331 was passed June 6, 1968. (Language since adopted into 3056);
Prior to this event, candidates and their families did not receive Secret Service protection;
Protection of a candidate/nominee is designed to maintain the integrity of the democratic process and continuity of Government.
Continuity of Goverment protection is for candidates.
https://www.secretservice.gov/protection/leaders/campaign-2024#:~:text=PL%2D90%2D331%20was%20passed,process%20and%20continuity%20of%20Government.