I figured it came from years of sucking limes.

Following a few links (I don't remember which links, but there it is), I find a possible beginning of a humorous series of blog posts (at least, humorous to those of us who lean left): Wingnut Mouth.

...I was struck once again by the presence of a characteristic I frequently notice amongst the winger noise machine -- an oddly distorted rictus of the mouth that makes it freeze in inappropriate positions, causing a smile to look not so much like the expression of joy as a symptom of gastric indigestion.

Heh! Indeed!