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 DonnellyJoseph (Joe) Simon DonnellyOvernight Energy: Trump taps ex-oil lobbyist Bernhardt to lead Interior | Bernhardt slams Obama officials for agency's ethics issues | Head of major green group steps down Trump picks ex-oil lobbyist David Bernhardt for Interior secretary EPA's Wheeler faces grilling over rule rollbacks 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 Sidney McCainMellman: Where are good faith and integrity? GOP senator says Republicans didn't control Senate when they held majority Pence met with silence after mentioning Trump in Munich speech MORE (Ariz.) and Lindsey GrahamLindsey Olin GrahamFBI’s top lawyer believed Hillary Clinton should face charges, but was talked out of it Overnight Defense: Graham clashed with Pentagon chief over Syria | Talk grows that Trump will fire Coats | Coast Guard officer accused of domestic terrorism plot Graham cursed at acting DOD chief, declaring himself his 'adversary' 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.