Hillary Clinton quickly apologized Friday after she invoked the 1968 assassination of Robert Kennedy as a way to explain why she is continuing her underdog bid for the Democratic nomination.

The former first lady made the original comments during an interview with the Sioux Falls Argus Leader editorial board, as she attempted to deflect calls for her to bow out of the race.

"My husband didn't wrap up the nomination in 1992 until he won the California primary somewhere in the middle of June," she said. "We all remember Bobby Kennedy was assassinated in June in California. I don't understand it."

Obama spokesman Bill Burton said the remark "was unfortunate and has no place in this campaign."

Clinton later addressed the comment, saying "I regret that if my referencing that moment of trauma for our entire nation and particularly for the Kennedy family was in any way offensive -- I certainly had no intention of that whatsoever."

She said, rather than hinting that she was holding out for the unspeakable to happen, she was merely trying to make the point that past Democratic nominating contests have lasted well into June. "That's an historic fact," she said.

She said the Kennedy family has "been much on my mind" lately because of the medical condition of Sen. Ted Kennedy, who was recently diagnosed with a brain tumor.

Clinton added: "I'm honored to hold Senator (Robert) Kennedy's seat in the United States Senate from the state of New York and have the highest regard for the entire Kennedy family."

Robert Kennedy Jr. released a statement saying it was clear Clinton was only talking about the political time frame.

"I understand how highly charged the atmosphere is, but I think it is a mistake for people to take offense," he said.

Even the Argus Leader's executive editor released a statement defending Clinton's response.

Obama foreign policy adviser Susan Rice told FOX News she would "take the Clinton campaign at their word" that she wasn't trying to suggest any correlation between Kennedy's assassination and "anything to do with Barack Obama."

She said it was "an unfortunate statement."

Clinton has previously referenced the 1968 Democratic primary campaign, and Kennedy's run, without directly mentioning his assassination.

"I remember what happened in the California primary in 1968 as Senator Kennedy won that primary," she said May 7 in West Virginia. "We traditionally have gone longer than you've seen in the last couple of cycles."

Clinton was in South Dakota Friday ahead of the state's June 3 primary.

Despite Obama's wide delegate lead in the race, Clinton told the editorial board Friday she's still "vigorously campaigning" and that she's been under unfair pressure to drop out ever since the Iowa caucuses Jan. 3.

Click here to watch the video of Clinton's meeting with the Argus Leader editorial board.

FOX News' Aaron Bruns contributed to this report.