I haven’t found a lot of polling on that race, but it looks like from the Real Clear Politics site that it was always tight even before Mourdock’s comments, which it really shouldn’t have been. I suspect Lugar and establishment Republicans were backbiting Mourdock throughout the race and making it hard for him to gain traction.
I remember seeing this button on eBay in 2012, showing the RINO influence in Indiana. Elect Willard for President, Pence for Governor and Donnelly (not Mourdock) for Senator. The Lugarites absolutely were looking to sabotage Mourdock after his decisive primary victory.