I would certainly go with exurbs. Remember back in the '60s, '70s and '80s when GOP strength was fueled by the suburbs? A huge chunk of those types of people have moved to exurbs. Many farmers are Republicans, but a fair amount would break from the party over things like ethonal subsidies (see Iowa county results in 2008). Also, as has been said, many small towns aren't as conservative as you'd think. Once you take into account ancestrally Democratic places like Elliot County, KY (even if most of these places turned on Obama), small college towns and areas like WI/MN or New England, where the small towns are actually the most left-leaning ... Well, you're left with exurbs.
Madison, Duluth, and Boston/Cambridge bid adeiu.
As for OP, I went with farms. The farming community is represented by Republicans in the legislature in an incredible way. The GOP has been in their interests for a long time, and if you look at areas like central Illinois and Indiana, you see that these areas haven't jumped parties at all. Where you have the GOP shift has been in southern Illinois and Indiana and Appalachia and swaths of Missouri and Arkansas where farming is not as big and mining is.
Poll
