Just this past weekend, I said this same question should be raised - "After almost 20 years in the Senate, what piece of legislation are you most proud to have sponsored or co-sponsored?" I would really like this question to be asked...repeatedly until he answers it.
Be careful with the wording. Many popular bills have scores of 'co-sponsors' Ask about author or lead sponsor. Being co-sponsor is a cheap publicity stunt; an easy way to associate yourself with a popular bill. I wouldn't doubt that Kerry could find lots of bills he 'co-sponsored' despite his complete lack of leadership.