He needs a southerner on the ticket. Going with Thune will show that Romney doesn't give a **** about the South.
This is why he should go with Christine O'Donnell.
Obama has not had a single Southerner (black, white, or otherwise) in his cabinet -- probably the first President ever to show so much contempt for a region that holds 30% of the U.S. population. I think even Lincoln had a Southerner or two (wasn't the slaveowning Blair the Postmaster General?) and he was in war against the South.
Steven Chu is from Missouri.
Also, in cabinet-level departments:
Lisa P. Jackson (EPA administrator) grew up in New Orleans
Ron Kirk (Trade representativE) is from Austin
Austin Goolsbee (the previous Chairman of the Council of Economic Advisors) is from Waco.