Mourdock beat incumbent Sen. Dick Lugar (R-Ind.) in the Republican primary earlier this year and is facing a tougher than expected challenge from Democratic Rep. Joe DonnellyJoe DonnellyDemocrats block energy spending bill over Iran amendment GOP blocks Obama sanctions czar Indiana GOP divided over Senate primary MORE (Ind.). Lugar decided not to help or campaign for Mourdock after he was defeated in the primary.
Other Republican senators, such as John McCainJohn McCainOvernight Energy: Clinton takes on former coal industry CEO The Trail 2016: Indiana gets ugly on GOP judgment day Ex-McCain aide: I'm with Clinton over Trump MORE (Ariz.) and Lindsey GrahamLindsey GrahamGraham: GOP has 'lost its way' on Trump Troops question rules for ISIS medal The beginning of the end for Ted Cruz MORE (S.C.), have gone to Indiana to campaign for Mourdock.
The Indiana seat is viewed as a critical win in order for Republicans to get control of the Senate.
Democrats are defending 23 Senate seats compared to just 10 Republican seats, but thanks to good recruiting and a few GOP fumbles, Democrats have been able to stay competitive in states — such as Indiana, Missouri, North Dakota and Maine — that were once thought of as potentially easy pick-ups for Republicans.