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 DonnellySenators to Obama: Make 'timely' call on Afghan troops levels This week: GOP lawmakers reckon with Trump Democrats block energy spending bill over Iran amendment 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 McCainWhy a power grid attack is a nightmare scenario Senate fight brews over Afghan visas Trump: Illegal immigrants treated better than veterans MORE (Ariz.) and Lindsey GrahamLindsey GrahamSenators to Obama: Make 'timely' call on Afghan troops levels Senate amendments could sink email privacy compromise Trump: Romney 'walks like a penguin' 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.