https://abcnews.go.com/Politics/presidential-candidate-secret-service-protection-factors-analysis/story?id=62513154
After Democratic nominee Robert F. Kennedy was assassinated at the Ambassador Hotel in Los Angeles following his winning the California primary in 1968, Congress changed the law to authorize Secret Service protection for major presidential and vice presidential candidates and their spouses within 120 days of the general presidential election.
The term "major presidential and vice presidential candidates" means those identified as such by the secretary of Homeland Security after consultation with an advisory committee consisting of the speaker of the House of Representatives, the minority leader of the House of Representatives, the majority and minority leaders of the Senate and one additional member selected by the other members of the committee.
A candidate has to hit several markers to show they are a "major presidential candidate." According to the Congressional Research Service, that includes:
They are a publicly declared candidates.
They are actively campaigning nationally and are contesting at least 10 state primaries.
Are pursuing the nomination of a qualified party, one whose presidential candidate received at least 10% of the popular vote in the prior election.
Are qualified for public matching funds of at least $100,000, and have raised at least $10 million in additional contributions.
Have received by April 1 of the election year an average of 5 percent in individual candidate preferences in the most recent national opinion polls by ABC, CBS, NBC, and CNN, or have received at least 10 percent of the votes cast for all candidates in two same-day or consecutive primaries or caucuses.
Additionally, like with Obama, threats can play a determining factor as to whether a candidate should be eligible for protection. While the threat dynamic is not a stipulated point, it is reviewed and weighed as a supporting factor.
Typically, though, the threat dynamic follows candidates who are "out there" the most – and the candidates "out there" the most are usually major candidates.
(MORE: Should the Secret Service Protect Candidates Online?)
Once they hit those markers, the secretary of Homeland Security consults with the advisory committee and one additional member selected by the other members of the committee (usually from the Secret Service) and determines if a candidate is eligible for Secret Service protection.
If the candidate is eligible, they are notified of the committee's decision and asked if they would like protection.