Senator Hillary Rodham Clinton quickly apologised after citing the June 1968 assassination of Robert F Kennedy as a reason for her to remain in the race for the Democratic presidential nomination despite increasingly long odds.
"I regret that if my referencing that moment of trauma for our entire nation -- and in particular the Kennedy family -- was in any way offensive. I certainly had no intention of that whatsoever," the former first lady said on Friday.
The episode occurred as Clinton campaigned in advance of the June 3 South Dakota primary.
Responding to a question from the Sioux Falls Argus Leader editorial board about calls for her to drop out of the race, she said, "My husband did not wrap up the nomination in 1992 until he won the California primary somewhere in the middle of June, right? We all remember Bobby Kennedy was assassinated in June in California. You know I just, I don't understand it," she said, dismissing the idea of abandoning the race.
Clinton said she did not understand why, given this history, some Democrats were calling for her to quit.
Her remark about an assassination during a primary campaign drew a quick response from aides to Democratic presidential front-runner Barack Obama.
"Senator Clinton's statement before the Argus Leader editorial board was unfortunate and has no place in this campaign," said Obama spokesman Bill Burton.
Clinton spokesman Mo Elleithee said the senator was only referring to her husband and Kennedy "as historical examples of the nominating process going well into the summer and any reading into it beyond that would be inaccurate and outrageous.