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 DonnellyWith GOP’s healthcare bill on ice, Dems go on offense Path to 60 narrows for Trump pick The Hill’s Whip List: Where Dems stand on Trump’s Supreme Court nominee MORE (Ind.). Lugar decided not to help or campaign for Mourdock after he was defeated in the primary.

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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 McCainWounded Ryan faces new battle Senate takes up NATO membership for Montenegro A great military requires greater spending than Trump has proposed MORE (Ariz.) and Lindsey GrahamLindsey GrahamCan Trump rebound after failure on healthcare bill? Senate takes up NATO membership for Montenegro Republicans giving Univision the cold shoulder: report 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.