Idiom and usage: Militate, not mitigate, against

In my morning paper, in the usual wonderful work by columnist Cindi Ross Scoppe explaining why S.C. government often fails to work, was this:

But our Legislature is far from ideal; the way it operates actually mitigates against legislators collectively fully understanding the operations of government.

It's a common misuse: mitigate against.

The idiomatic usage is militate against - militate meaning "to exert a strong influence." It works with "against" (substitute it in that sentence: "actually exerts a strong influence against" ...)

Mitigate means to make less severe and does not work with "against."

Bryan Garner, in Garner's Modern American Usage, says the misuse has become so common that it rates a three on his five-point language-change scale. But he still advises against using it.