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 DonnellyGinsburg health scare raises prospect of election year Supreme Court battle Watchdog accuses pro-Kavanaugh group of sending illegal robotexts in 2018 Lobbying world 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 McCainOvernight Defense: GOP lawmaker takes unannounced trip to Syria | Taliban leader pens New York Times op-ed on peace talks | Cheney blasts paper for publishing op-ed GOP lawmaker makes unannounced trip to northeastern Syria Meghan McCain after Gaetz says Trump should pardon Roger Stone: 'Oh come on' MORE (Ariz.) and Lindsey GrahamLindsey Olin GrahamSunday shows preview: 2020 candidates look to South Carolina Where do we go from here? Conservation can show the way Barr to attend Senate GOP lunch on Tuesday 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.