Democrat Joe Donnelly defeated Richard Mourdock for an open U.S. Senate seat in Indiana, one of Republicans’ must-win races in their effort to gain control of the chamber.

The contest between Mourdock, the state treasurer, and Donnelly, a three-term U.S. congressman, gave Democrats an opportunity to claim a long-held Republican seat. Mourdock, 61, an anti-tax Tea Party favorite, used an anti-Washington, D.C., message to defeat six-term Republican Senator Richard Lugar by 20 percentage points in the party primary in May.

Much of the race hinged on whether Mourdock could win over voters who supported Lugar for 36 years. Lugar declined to campaign for Mourdock. The senator’s spokesman, Andy Fisher, said Mourdock “perpetuated misleading statements” about Lugar during the primary campaign.

Further imperiling his candidacy, Mourdock said in a debate with Donnelly, 57, two weeks before the election that "even when life begins in that horrible situation of rape, that it is something God intended to happen.”

Donnelly’s campaign website called Mourdock’s statement part of a “troubling pattern of extreme positions.”

Mourdock stood by his position while apologizing for his “less than fully articulate use of words.” The fallout spilled into the presidential campaign as Democrats called on Republican presidential nominee Mitt Romney to denounce Mourdock’s comments and revoke his endorsement of the Senate candidate.

Romney’s campaign said he disagreed with Mourdock’s statement though he wouldn’t take back his endorsement.

Andrew Horning, the Libertarian candidate who hasn’t held elective office, may have also hurt Mourdock. He peeled off about 6 percent of the vote that might have gone for the Republican.

During a series of debates during the race, Horning mocked the wars in Afghanistan and Iraq as an effort to root out individuals who wear “exploding underpants.”