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.
Good point about co-sponsoring; however, it might be telling what he would claim to be "most proud of". Hasn't Kerry indicated that he was "proud" to vote AGAINST the $87 billion supplemental bill (after he voted for it)?
I guess I'm just wishing that his Senate record would get some exposure.