McConnell to campaign for Mourdock in Indiana's Senate race

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 DonnellyGreat Lakes senators seek boost for maritime system Liberal groups urge Schumer to reject Bayh for Banking gavel A dozen senators call for crackdown on Chinese steel MORE (Ind.). Lugar decided not to help or campaign for Mourdock after he was defeated in the primary.

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 McCainYes, let’s set politics aside on sage grouse conservation Democrats pounce on Cruz's Supreme Court comments Obama's right to tackle redistricting, but it won't be easy MORE (Ariz.) and Lindsey GrahamLindsey GrahamVulnerable GOP senator questions opponent's American heritage Trump on primary rivals who don't back him: 'I don't know how they live with themselves' The Trail 2016: Who is really winning? 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.