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 expressed regret over the remark, made it to the editorial board of a South Dakota newspaper, the Sioux Falls Argus Leader, when explaining that other races for the Democratic presidential nomination had lasted into June.

"My husband (Bill Clinton) 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 race in which the final nominating primaries are held on June 3 in South Dakota and Montana. The Democratic nominee will face Republican John McCain in the November election.

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

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

A spokesman said Clinton had simply been pointing out that Democratic campaigns in the past had continued into June and therefore people should not be pressuring her to withdraw.

'HISTORICAL EXAMPLES'

"She was simply referencing her husband in 1992 and Robert Kennedy in 1968 as historical examples of the nominating process going well into the summer. Any other reading is inaccurate," Clinton spokesman Mo Elleithee said.

Robert Kennedy Jr., son of the slain senator, said in a statement, "It is clear from the context that Hillary was invoking a familiar political circumstance in order to support her decision to stay in the race through June."

"I understand how highly charged the atmosphere is, but I think it is a mistake for people to take offense," said Kennedy, a Clinton supporter unlike some other members of his family, including Sen. Edward Kennedy, who back Obama.

There have been concerns about the safety of Obama, an Illinois senator who would be the first black U.S. president. He began began receiving Secret Service protection 18 months before the November election -- earlier than any other candidate has received increased security.

Obama has an almost insurmountable lead over Clinton in the race for the Democratic nomination. The former first lady has said she would continue until the last state has held its nominating contest.