By Thomas Ferraro

SIOUX FALLS, South Dakota (Reuters) - Hillary Clinton mentioned the June 1968 assassination of Robert Kennedy in explaining on Friday why she had resisted calls to end her White House bid, drawing a rebuke from Democratic front-runner Barack Obama's campaign.

Clinton, who later apologized, made the remark about Kennedy to the editorial board of the Sioux Falls Argus Leader newspaper when explaining that other races for the Democratic presidential nomination had lasted into June.

"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," she said.

"I don't understand it," Clinton said, referring to calls for her to pull out of the Democratic nominating race, which ends on June 3 with primaries in South Dakota and Montana.

Kennedy, brother of slain U.S. President John F. Kennedy and Massachusetts Sen. Edward Kennedy, was assassinated during the 1968 race for the Democratic nomination.

Clinton's comments drew a sharp response from the Obama campaign.

"Senator Clinton's statement before the Argus Leader editorial board was unfortunate and has no place in this campaign," said Obama campaign spokesman Bill Burton.

Clinton told reporters later, "I regret if my referencing of that moment of trauma for our entire country and particularly the Kennedy family was in any way offensive. I had no intention of that whatsoever."