"It's a decision the candidate has to request," said Secret Service spokesman Tom Mazur.
Federal law allows candidates to seek protection if they meet a series of standards, including public prominence as measured by polls and fund raising.
"A threat could be part of that equation," Mazur said, declining to reveal whether Kerry was the subject of specific threats.
As for Edwards, he said, "Protection of Senator Edwards has been authorized and the Secret Service is in discussions with the campaign as to when the protective detail will start."
"I expect to have an announcement on that soon," said Edwards spokeswoman Jennifer Palmieri.