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 DonnellyGOP loses top Senate contenders Pruitt sworn in as EPA chief Dem senator: I may face 2018 primary from Tea Party-esque progressives 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 McCainTrump names McMaster new national security adviser How does placing sanctions on Russia help America? THE MEMO: Trump's wild first month MORE (Ariz.) and Lindsey GrahamLindsey GrahamTrump’s feud with the press in the spotlight Senators eye new sanctions against Iran Republicans play clean up on Trump's foreign policy 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.