The GOP lawmaker principle: http://www.slate.com/blogs/weigel/2014/03/18/the_gop_lawmake...
Tl;dr is that only one person (the neo-Nazi perennial candidate) filed to run in the GOP primary in a safe-D seat, and state law precludes a write-in campaign. Democrats also occasionally get embarrassing nominees this way, especially in safe seats, e.g.
https://en.wikipedia.org/wiki/Kesha_Rogers https://en.wikipedia.org/wiki/Mark_E._Clayton