As a friend of mine said in an e-mail, paraphrasing Hugh Hewitt: John's McCain's a great American, a terrible senator, a lousy Republican, and possibly one of the worst singers in the world. The evidence of the latter is here.
One must ask the question: What was he thinking?
Actually, McCain's performance is pretty funny and makes a point about entertainers who (like Streisand) get involved in politics. It looks like something that might have been on Saturday Night Live a few years back (McCain is visibly younger in this clip.)