SurveyUSA surveys approval for all 100 Senators, and the Kansas Senators don't do well. Brownback comes in at 67, and Roberts at 71. James Inhofe is among the least popular Senator who will be up for re-election next year. Beating him would be a real win.
Looking in more detail at the Kansas Senators, both saw major drops in popularity. Alas, the shift is mostly among Conservatives and Liberals, moderate and Independent approval has been flat.
That said, Roberts and Brownback have both trailed Governor Sebelius in job approval, and it's likely that she could make a race against either into one to watch.