Donnelly promised supporters gathered in Indianapolis on Tuesday night he would go to Washington as a senator in the mold of Republican Richard Lugar and former Democratic U.S. Sen. Evan Bayh.
"I'm not going there as one party's senator or the other party's senator, I'm going there as your senator to work for your families," he said. "I'm the hired help, and I can't wait to get to work."
Mourdock conceded defeat, tearing up at times as he said he was worried for the nation.
"Tonight, my own disappointment aside, my concern for this nation grows greater," he said. "That's not meant as slap on Mr. Donnelly; I wish him well."
The victory was a coup for Democrats, who had been waiting years for a shot at the seat. It had been held since 1977 by Lugar, who was defeated by Mourdock in a bitterly fought primary.
Even a year ago, Lugar seemed a safe bet to win a seventh term, despite widespread conservative anger with the veteran statesman's votes on divisive legislation and his support for President Barack Obama's Supreme Court nominees. But questions about Lugar's residency combined with a flood of outside spending by groups such as the anti-tax Club for Growth carried Mourdock to a 20-point victory in the May primary.
Democrats pounced on the opportunity as Mourdock made a series of quick missteps that alarmed more moderate Republicans. In a series of interviews the day after his primary victory, Mourdock said compromise should consist of Democrats bowing to Republican demands and stood by tea party views popular with the most conservative voters, but not many others.
"To me the highlight of politics, frankly, is to inflict my opinion on someone else," he told MSNBC the day after the primary.
Mourdock later tried to tack back toward the middle with declarations that he could work with Democrats, but he stumbled again in a televised Oct. 23 debate when he explained his opposition to abortion except in cases in which the mother's life is in danger.
"I struggled with it myself for a long time, but I came to realize that life is that gift from God. And, I think, even when life begins in that horrible situation of rape, that it is something that God intended to happen," Mourdock said.
Republicans, including presidential nominee Mitt Romney, initially distanced themselves from Mourdock but later walked their criticism back, with many saying they didn't agree with his statement but supported Mourdock's candidacy.
Democrats spent millions of dollars flooding the airwaves with those comments and other statements by Mourdock in a bid to attract disillusioned Lugar supporters.
Mike Murphy, a former Republican state lawmaker and veteran operative, cautioned that Mourdock's loss should not be read as a repudiation of the tea party, but one man's incredible efforts to wrench defeat from the jaws of victory.
"What happened with Mourdock was personal self-destruction, it wasn't a complete repudiation of conservative ideas in the Republican Party," he said.
Exit polling showed Indiana voters picking the economy as their top issue, driving victories for other Republicans. But it also showed women breaking heavily for Donnelly.
For all the Mourdock campaign said about the comment not mattering to voters and arguing that the electorate was more concerned with the federal health care overhaul and federal spending, some voters said it still weighed on their minds in the voting booth.
Kaye Young, 78, of Indianapolis voted for Lugar in the primary and said she thought it was "a shame they kicked him out."
She said Richard Mourdock "irritated the tar out of me" with his comment on rape. But she still voted for him.
"I don't want a Democrat in there," she said.
Mourdock's comment came to be the defining moment of the race. A Howey/DePauw University Battleground poll taken Oct. 28-30 showed Donnelly opening a double-digit lead over Mourdock.
"Candidates really matter in Indiana. They (voters) want a good, common-sense approach. They don't like candidates too far in either direction," said Christine Matthews, a veteran Republican pollster who conducted the Howey/DePauw poll with Democratic pollster Fred Yang.
Indiana's Senate battle was the most expensive the state ever has seen, topping $25 million spent on air by outside groups and the campaigns.
Donnelly now becomes the new standard-bearer for Indiana's Democrats, whose statewide successes almost exclusively have stemmed from the Bayh family. Mourdock, meanwhile, joins the ranks of tea party candidates who ousted moderate Republicans in primaries but could not find enough support among the general electorate.