Remember that the primary has already passed here. Tim Johnson already won it, which likely means he'll get to essentially appoint his successor.
Furthermore, this district is pretty marginal. Tim Johnson running scared most of the high-profile Dems away, and the primary was won by perennial candidate and general nut David Gill, which means that the Republican is pretty much assured of winning.
Larry Buschon was seen as a nut in IN-08 and raised almost no money, but won easily by default when Ellsworth retired. Gill got 42% in 2006 in a district that was R+6. The new district is D+1. A swing of eight points and DCCC money could easily push him over the top.