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 DonnellyTrump’s vow on Medicare in doubt after HHS choice Red-state Dems face tough votes on Trump picks Red-state Democrat: I'll oppose Trump's health chief 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 McCainPentagon should have a civilian chief to give peace a chance McCain to support waiver for Mattis, Trump team says Senators crafting bill to limit deportations under Trump MORE (Ariz.) and Lindsey GrahamLindsey GrahamPentagon should have a civilian chief to give peace a chance Lawmakers eye early exit from Washington Senate passes college anti-Semitism bill 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.