First the candidates have to request it. They must write a letter an official letter to the secretary of DHS and ask for the protection. DHS doesn't just offer the protection because of the optics involved.
1. Are they publicly announced and have they filed with the federal elections commission?
2. Are they actively campaigning on a national basis? And has a threat assessment indicating possible threats to them been done?
They then decide if protection is warranted.
Even if the advisory committee denies it. The secretary of DHS can come back and initiate coverage.