They didn't "defame" him.

They give their personal opinions that he was crummy officer nearly forty years earlier.

I haven't heard that they slandered him, as by lying - which is the essence of defamation.

Like so many here, people swayed by the ad couldn't tell the difference between fact and opinion, a straw man argument, cherry-picked testimony, and (perhaps) the relevance of behavior forty years ago, long before Kerry's more than competent career in the U.S. Senate.

Maybe Kerry was a bad choice. Maybe not. In either case, his failure to live up to the idealized movie image of a naval officer in his twenties shouldn't have carried any decisive weight.

Or should it have?

Opinions. Everyone's got plenty.