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 DonnellySenators introduce dueling miners bills Government to begin calling Indiana residents Hoosiers Pence meets with Kaine, Manchin amid Capitol Hill visit 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 McCainSenate committee to vote Monday on Tillerson Trump fails to mention Clinton in inaugural address Hillary Clinton under microscope at inauguration MORE (Ariz.) and Lindsey GrahamLindsey GrahamThe Hill's 12:30 Report Graham: Trump would make mistake in not punishing Russia Graham to vote for Trump’s EPA pick 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.