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 DonnellyJoseph (Joe) Simon DonnellyDemocrats now attack internet rules they once embraced Dem group launches M ad buy to boost vulnerable senators Senate rejects Trump immigration plan MORE (Ind.). Lugar decided not to help or campaign for Mourdock after he was defeated in the primary.

Donnelly has argued that Mourdock, a Tea Party favorite, is too extreme and conservative for the centrist voters in Indiana, who had elected Lugar by a landslide in the past.

Other Republican senators, such as John McCainJohn Sidney McCainLawmakers worry about rise of fake video technology Democrats put Dreamers and their party in danger by playing hardball Trump set a good defense budget, but here is how to make it better MORE (Ariz.) and Lindsey GrahamLindsey Olin GrahamCongress punts fight over Dreamers to March Pence tours Rio Grande between US and Mexico GOP looks for Plan B after failure of immigration measures 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.