McCain responded to reporters' questions this month after Secret Service chief Mark Sullivan made the eyebrow-raising disclosure that the senator was going solo at a hearing of the House Appropriations Committee, the body that sets the budget for the agency.
"Statutorily, he is not required to take protection," Sullivan explained. "As far as an actual request, we have not gotten one. We have no involvement at this point."
Presidential candidates must ask for protection to be considered, but the criteria limit the recipients of protection to a precious few. Presidents, vice presidents and former holders of those offices all receive protection, with their families.
Protection used to be for a lifetime, but under a new law, President George W. Bush would become the first president to lose protection after 10 years.
Those covered also include foreign heads of state traveling in the United States -- including Pope Benedict XVI in his capacity as head of the Vatican -– and anyone the president directs to be protected.
The agents sweep locations with dogs, scans visitors using magnetometers and visually scour the crowd for potential threats.
It's an expensive, labor intensive effort. The cost to protect Vice President Dick Cheney in the six months after he leaves office next year -- including agents, transportation, advance visits and the rest of it -- is expected to run to $4 million.
Candidates are given protection on a case-by-case basis after consideration by a group called the Bipartisan Advisory Committee, which includes the majority and minority leaders of both houses of Congress and one other member they select. The committee considers popularity of the candidate, as gauged by polls, and fundraising. Final decisions are made by the secretary of the Department of Homeland Security.
The Secret Service, now a $1.5-billion-a-year agency, was founded in 1865 as an investigative agency to curb counterfeiting. It didn't begin protecting presidents until 1901, after the assassination of President McKinley.
Protection for presidential candidates has increased since the assassination of Robert F. Kennedy, who appeared headed to victory in the Democratic nomination after winning California before he was gunned down in Los Angeles' Ambassador Hotel in 1968.