I'm guessing since she didn't get any grief about it before, she didn't really give it much thought and just said it again today. That's what happened when you're being filmed.
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Though she has now apologized for that very strange and tasteless comment to the Argus-Leader, this was not the first time she's said it. This from her interview with TIME Managing Editor Richard Stengel, published March 6:
TIME: Can you envision a point at which--if the race stays this close--Democratic Party elders would step in and say, "This is now hurting the party and whoever will be the nominee in the fall"?
CLINTON: No, I really can't. I think people have short memories. Primary contests used to last a lot longer. We all remember the great tragedy of Bobby Kennedy being assassinated in June in L.A. My husband didn't wrap up the nomination in 1992 until June. Having a primary contest go through June is nothing particularly unusual.
Her excuse now is that the Kennedys have been "much on my mind these days" with the illness of Senator Edward Kennedy, but that doesn't explain what brought it to mind more than two months ago.
UPDATE: H/T to commenter Elvis Elvisberg, who notes that even the reference to her own husband's campaign was wrong. Bill Clinton's primary race was effectively over in March.
http://www.time-blog.com/swampland/2008/05/hillarys_bizarre_rfk_comment.html