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 DonnellyRepublicans may regret restricting reproductive rights Sanders traveling to Iowa, Indiana to pitch Biden's spending package Supreme Court battle could wreak havoc with Biden's 2020 agenda 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 McCainWhoopi Goldberg signs four-year deal with ABC to stay on 'The View' Collins to endorse LePage in Maine governor comeback bid Meghan McCain: Country has not 'healed' from Trump under Biden MORE (Ariz.) and Lindsey GrahamLindsey Olin GrahamTrump pushes back on book claims, says he spent 'virtually no time' discussing election with Lee, Graham The Hill's 12:30 Report - Presented by Facebook - Biden meets with lawmakers amid domestic agenda panic The Hill's Morning Report - Presented by Alibaba - House Democrats plagued by Biden agenda troubles 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.