By Monica Davey and Trip Gabriel
INDIANAPOLIS — The morning after Senator Richard G. Lugar, in his 36th year in office, was overwhelmingly defeated in a Republican primary election, this state awoke on Wednesday to another surprise: A new battle, now likely to be far fiercer and costlier than once expected, was already brewing over the seat he leaves behind.
Democrats were casting the general election fight as a referendum on whether moderates should still have a place in Washington, while Tea Party organizers said it would be seen as a national test of the movement’s enduring strength.
Democratic leaders, who had doubted their odds against Mr. Lugar, a Republican so moderate that even the leaders admitted that plenty of Democrats liked him, sounded giddy about their November opponent: Richard E. Mourdock, a Tea Party-supported Republican who seized a remarkable 61 percent of the vote in part by denouncing bipartisanship and pledging to an unwavering conservative approach.
“Democratic donors across the country are going to see this as a prime pickup opportunity,” said Matt Canter, a spokesman for the Democratic Senatorial Campaign Committee, who added that the Indiana seat would fall among five top Republican-held seats being targeted in the fight for control of the Senate.
Labor leaders, too, said they saw an opportunity now in Indiana. “We’re all ramping up our plans as we speak,” said Nancy J. Guyott, president of the Indiana A.F.L.-C.I.O.
National conservative groups, some of which had poured more than $3 million to benefit Mr. Mourdock in the primary, were poised to send still more if needed. The number of such outside groups also appeared likely to grow if the contest here, against Representative Joe Donnelly, a Democrat, appears truly competitive — a notion some conservative leaders remained skeptical about, given Indiana’s Republican leanings.
“It’s a big race because a lot hinges on our success,” said Brendan Steinhauser, director of federal and state campaigns at FreedomWorks, which trains Tea Party members and which spent about $850,000 in Mr. Mourdock’s victory and plans to be similarly involved in the general election.
“If Mourdock were not to win,” Mr. Steinhauser said, the gloating would come not just from Democrats but establishment Republicans, pointing to the Tea Party. “They would want to blame that on us — ‘See, we told you so,’ ” he said.
By Wednesday, the outlines of a new political battle were emerging, with Democrats trying to paint Mr. Mourdock as a far-right candidate with little appeal for independents or moderate Republicans, and conservatives portraying Mr. Donnelly as a typical Democrat.
The Club for Growth, which had spent money on television and radio commercials against Mr. Lugar in the primary and said it would contribute more, if needed, in the general election, said Mr. Donnelly was “an economic liberal who votes in lock-step” with Democratic leaders.
Mr. Donnelly, who was elected in 2006 to represent a northern Indiana district, described himself as among the most conservative Democrats in the House in a moment when, he said, voters are looking for something different than they were in 2010. “Right now, it’s not about fire and brimstone,” he said. “It’s about jobs and the opportunity for your family to succeed.”
Although President Obama won Indiana in 2008, the state has long been a place where Republicans do well, and Mr. Obama is considered unlikely to win here again. None of it would seem to be fertile ground for a Democratic Senate bid, and some conservative leaders said they remained unconvinced that Mr. Mourdock would have any trouble in November.
But Dan Parker, the chairman of the Indiana Democratic Party, described Mr. Mourdock as an “extreme Tea Party candidate,” who would not appeal to a general election audience. “Dick Lugar was the mainstream Republican,” he said. “Indiana is not crazy conservative.”
Senator Chris Coons of Delaware, a Democrat, who rose from nowhere to be elected in 2010 after Republicans there rejected a popular moderate and nominated Tea Party favorite Christine O’Donnell, said he reached out to Mr. Donnelly Tuesday night after Mr. Lugar’s defeat.
“I think there’s a very good chance he could be the Chris Coons of 2012,” the senator said.
State Republican leaders, calling on Wednesday for party unity following Mr. Lugar’s loss, stood beside Mr. Mourdock on a stage here and seemed eager to play down his Tea Party ties and emphasize his traditional Republican credentials. Complicating the efforts, Mr. Lugar, who was not in attendance at the gathering, issued a sharp statement condemning what he suggested was a rising trend of rejecting political independence and bipartisan conversation.
“He comes right out of the heart, right out of mainstream of our party, and I think that was really, among many, his longest single suit in the huge win that he had yesterday,” said Gov. Mitch Daniels, the governor, who had endorsed Mr. Lugar and had previously said he viewed Mr. Mourdock as a friend.
Indeed, Mr. Mourdock is in his second term as state treasurer, and he has been known for appearing at local Republican events and county dinners for years; when he announced his bid for the senate, he had a surprising majority of endorsements from the party’s county chairmen and chairwomen around the state.
“The first label they’re going to try to put on me is that Mourdock is this wild-eyed Tea Party guy,” Mr. Mourdock said. “But as the governor said, I’ve been swimming in the pool of Republican politics a long time,” he said, growing choked up as he described his love for the party.
Jonathan Weisman contributed reporting from Washington, and Steven Greenhouse from New York.