Sioux Falls, S.D. ? Sen. Hillary Rodham Clinton quickly apologized Friday after citing the June 1968 assassination of Robert F. Kennedy in defending her decision to keep running 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.

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 didn’t 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.