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 DonnellyOvernight Finance: Trump threatens NAFTA withdrawal | Senate poised for crucial Puerto Rico vote | Ryan calls for UK trade deal | Senate Dems block Zika funding deal Senate Democrats block Zika agreement ahead of recess Post Orlando, hawks make a power play 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 McCainBush World goes for Clinton, but will a former president? GOP senator: Trump could lose Arizona Senate panel passes bill that would create 4K visas for Afghans MORE (Ariz.) and Lindsey GrahamLindsey GrahamOvernight Defense: US blames ISIS for Turkey attack | Afghan visas in spending bill | Army rolls up its sleeves Senate panel passes bill that would create 4K visas for Afghans Trump: Rivals who don't back me shouldn't be allowed to run for office 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.