Presidential Candidate and Nominating Convention Security [Updated July 28, 2020] [open pdf - 452KB]
From the Introduction: "In 1968, Congress authorized U.S. Secret Service (USSS) protection of presidential candidates following the assassination of Democratic Party presidential candidate Senator Robert F. Kennedy. [...] There are two categories of candidates who receive USSS protection: (1) candidate protectees, who receive protection prior to receiving their political party's nomination; and (2) nominee protectees, who receive protection following their party's nomination. The USSS is statutorily required to protect major presidential and vice presidential candidates within 120 days of the general presidential elections, as well as their spouses (18 U.S.C. Section §3056(a)). This In Focus discusses protection of presidential candidates--both before and after the party nominating conventions--and provides an overview of funding for security operations at the nominating conventions."
CRS In Focus, IF11555
Congressional Research Service: https://crsreports.congress.gov/