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 DonnellyLawmakers sport LSU gear at baseball game in honor of Scalise Senate votes to continue arming Saudis As Yemenis suffer the consequences Overnight Defense: Mattis defends Trump budget | Senate rejects effort to block Saudi deal | Boeing to cut 50 executive jobs 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 McCainCongress needs to support the COINS Act GOP’s message on ObamaCare is us versus them Frustrated Dems say Obama botched Russia response MORE (Ariz.) and Lindsey GrahamLindsey GrahamSenate panel questions Lynch on alleged FBI interference The Hill's Whip List: Senate ObamaCare repeal bill Judiciary Committee to continue Russia probe after Mueller meeting 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.