Trump expressed this view at a campaign event, noting he didn't care for "losers," referring to McCain's 2008 election loss”.
There doesn't seem to be any verification that shitstain was referring to McCains election loss as he ended his sentence on a open from other quotes I read.
I remember seeing an interview where he was saying this line and there wasn't a comment about McCains election, just the comment about John's war experience. I could be wrong.