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Sen. Richard Lugar, the third longest-serving member of the Senate, went down to a primary defeat tonight to his Tea Party-backed opponent in the Republican primary.

State Treasurer Richard Mourdock, backed by tea partiers and conservative campaign groups outside the state, ousted Lugar in Indiana's GOP primary, the Associated Press projected. With votes tallied in more than three quarters of Indiana counties, Mourdock led 61 percent to 39 percent.

Mourdock will face Democratic Rep. Joe Donnelly in November.

In Lugar, the Senate would lose one of its few remaining members with a habit of bipartisanship. In Mourdock, Lugar has been unseated by a mild-mannered, twice-elected statewide official who wants to eliminate five federal departments and cut more spending than House Budget Chairman Paul Ryan, R-Wis., would.

"I hope that Richard Mourdock prevails in November so that he can contribute to that Republican majority in the Senate," Lugar said in his concession speech. "We are experiencing deep political divisions in our society right now. And these divisions have stalemated progress in critical areas. These divisions have stalemated progress in critical areas. But these divisions are not insurmountable. I agree that people of good will, regardless of party, can work together for the benefit of country."

Mourdock began his acceptance speech by leading a round of applause for Lugar.

"When I began this campaign, Sen. Lugar was not my enemy. He is not now my enemy; he will never be my enemy. He was, simply, over the last 15 months, my opponent," Mourdock said. "Hoosiers want to see Republicans inside the U.S. Senate take a more conservative track."

President Obama lamented Lugar's defeat in a statement released to press. "While Dick and I didn't always agree on everything, I found during my time in the Senate that he was often willing to reach across the aisle and get things done. My administration's efforts to secure the world's most dangerous weapons has been based on the work that Senator Lugar began, as well as the bipartisan cooperation we forged during my first overseas trip as Senator to Russia, Ukraine and Azerbaijan," Obama said. A still photo of Lugar appeared in a 2008 Obama campaign ad, promoting Obama's bipartisan work on nuclear nonproliferation. Lugar suffered criticism over the ad in his run against Mourdock.

Throughout the campaign, Mourdock walked the line between attacking Lugar and showing him deference as a long-serving statesman. In their lone televised debate, Mourdock was reluctant to pounce on his opponent. "I can't attack this grandfatherly figure in Republican politics," he later explained to ABC News in a phone interview. Mourdock decided to run, he said, because members of the Indiana GOP asked him to-a request that surprised Mourdock, given Lugar's long tenure.

Lugar's loss made history. Among senators who had served at least six terms, only one had lost in a primary before Lugar: Kenneth McKeller, D-Tenn., who joined the Senate in 1917 and lost to Democratic primary challenger Al Gore, Sr. in 1952. Only 22 senators in history served as long as Lugar has of 1,931 total, according to the Senate historian.

Lugar currently ties Utah's Orrin Hatch as the Senate's longest-tenured Republican. Hatch is also facing a conservative primary challenge in 2012.

Mourdock's win was expected by political operatives in D.C. and Indiana after an expensive campaign in which outside groups flocked to the Hoosier State. A total of 12 groups spent $4.6 million, only one of them based in Indiana.

If raw spending had decided the race, Lugar would have won. As of mid-April, Lugar had spent $6.7 million defending himself, to Mourdock's $2 million. Outside groups spent more heavily in favor of Mourdock.

Tea partiers and conservative groups replayed their successful 2010 playbook to defeat Lugar, the first establishment casualty of 2012. The anti-Lugar charge was led by the Club for Growth and FreedomWorks, which routinely pick inexpensive states and vulnerable Republican incumbents, attacking them for moderate votes. In Indiana, that meant Lugar's votes for TARP, against an earmarks moratorium, and in favor of Supreme Court justices Sonia Sotomayor and Elena Kagan.

Even after the tea party wave of 2010, Lugar failed to heed warnings of danger, according to one Republican strategist.

"In the beginning of 2011, Sen. Lugar's campaign was warned of what was coming at them, and obviously other Republicans, Orrin Hatch and [Maine GOP Sen.] Olympia Snowe, who had both hired top campaign staff, had heeded that warning and were prepared," the strategist said.

Lugar had his own problems. He was briefly  ruled ineligible to vote in the county where he was registered, after selling his home in the late 1970s and moving to Washington, D.C. He  repaid the Treasury after using taxpayer money for hotel stays during trips back to Indiana. That saga allowed Lugar's opponents to characterize him more aggressively as casting moderate votes because he was "out of touch" with Indiana.

"I think what you're seeing is a confluence of factors that are challenging for Sen. Lugar, and in some ways what's going on in Indiana is a microcosm of what's going on nationally in the Republican Party, with a few elements added in," former Indiana Democratic senator Evan Bayh told ABC News last week.

"Indiana is a conservative state," Bayh said. "I'm not surprised that some of these outside groups would choose to get involved."

Mourdock's win certainly signifies that the Republican Party has continued to grow more conservative. Where Lugar voted with Democrats to advance the DREAM Act and worked with the Obama administration to push the New START arms-reduction treaty through the Senate, Mourdock is as conservative and ideological as they come.

"Let's do away with the Department of Education, Energy, Commerce, Housing and Urban Development," Mourdock told ABC News in an April phone interview, and he has also has proposed ending the IRS. Mourdock has suggested that Paul Ryan's budget doesn't go far enough, and he released his own rough plan last year to shrink spending by $7.6 trillion in 10 years (Ryan's would reduce it by $5.5 trillion, according to the Congressional Budget Office).

Perhaps most significantly, Mourdock outspokenly opposes bipartisan compromise. "Bipartisanship has brought us to the brink of bankruptcy," he told ABC. "We don't need bipartisanship, we need application of principle."

While some 2010 tea-party candidates showed a lack of campaign discipline and political skill, Mourdock should prove more formidable. "For lack of a better comparison, he's not [failed Delaware Senate candidate] Christine O'Donnell, who just appeared out of nowhere. He's a two-time-elected statewide candidate, which means he's just more substantive," Bayh told ABC.

But Democrats have held back their opposition research on Mourdock in the hopes that he would win, and on Wednesday, Indiana voters can expect the start of a barrage of anti-Mourdock attacks by Democrats.

Mourdock's win might give Democrats a new chance to win Indiana's Senate seat in November. Donnelly's campaign says its internal polling has shown him performing far better against Mourdock than against Lugar. Majority PAC, the Democratic Senate-focused super PAC, spent money to help Mourdock's primary bid.

A GOP strategist acknowledged that, with Mourdock's win, Republicans would have to keep a closer eye on the race. Though, with Indiana solidly red in recent statewide elections, the party should feel good about its chances to keep Lugar's seat within the GOP ranks.

If Mourdock wins in November, he'll push the Senate's GOP conference further to the right, and he'll join a growing cadre of tea-party senators who have clashed at times with GOP leaders.

Asked in April how he would handle pressure to fall in line, Mourdock recounted meeting with conservative Sen. Jim DeMint, R-S.C., before finalizing his decision to run.

"He reached down, he's a tall guy, and put a hand on either shoulder, and said, 'Richard, you get me four or five more true conservatives, and we've just changed the leadership of the United States Senate,'"Mourdock said. "He said that doesn't necessarily mean we even change the people, but you get me four or five more true conservatives, and we've just changed the way they're gonna see things because of our numbers."