What's the history of presidential candidates selecting their primary rivals as running mates?

I wanted to emphasize that I'd like examples where the eventual running mate was the *most* prominent rival in the primaries. Looking back through the past couple of decades, though, I guess there weren't that many primaries that were as strongly contested as Obama/Clinton or even Romney/Santorum, so the idea of "most prominent rival" might be more debatable--or even nonexistent--in some cases.