"Teach only what you know"?
misterskank is quite the lightning rod with his comments.
The notion that one might "know" their subject content better than why one is voting for a particular candidate is itself invalid.
I know how to teach Joseph Conrad's Heart of Darkness. Do I "know" how he represents colonial racism any more than I "know" why to vote against George W. Bush or his ideological heirs? I'm much more convinced that I know the latter more than the former.
The title of the article is a red herring. What a teacher can do or communicate is more important than what one "knows". I can teach critical thinking. I do not "know" critical thinking.
However, keeping one's political views a secret as a teacher is a lot of fun. For me, that's the only defensible reason for keeping politics out of the classroom.