INDIANAPOLIS (Reuters) - Senator Richard Lugar, a 35-year veteran of the Senate and leading foreign policy voice, was defeated in the Indiana Republican primary by a Tea Party-backed challenger on Tuesday, the first Senate incumbent ousted in the 2012 election year.

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Lugar conceded defeat to challenger Indiana state Treasurer Richard Mourdock, who attacked the soft-spoken Senator for votes in support of Democratic President Barack Obama.

“My public service is not concluded,” Lugar told supporters in Indianapolis. “I look forward to what can be achieved in the Senate in the next eight months despite a very difficult national election atmosphere.”

The defeat of Lugar gives a boost to the Tea Party movement, which wants to force deep cuts in government spending and reduce the size of the federal government.

The outcome also gives Democrats an unexpected opportunity to win a Republican-held seat in November’s elections. Democrats are clinging to a 53-47 advantage in the Senate but have many more incumbents standing for re-election than Republicans.

When Lugar, 80, last ran for re-election in 2006, he was seen as so invincible that Democrats did not field an opponent.

But the soft-spoken senator, who is the senior Republican on the Foreign Relations Committee, saw the atmosphere of Congress change dramatically in recent years to gridlock.

After the arrival of the conservative insurgent Tea Party movement on the political scene, Lugar’s long track record of bipartisanship and foreign policy expertise were out of fashion.

His votes for Obama’s Supreme Court appointees Sonia Sotomayor and Elena Kagan riled conservatives, as had the fact that he had not lived in Indiana since 1977.

Mourdock said Lugar had lost touch with Indiana Republicans, who were looking for change. Conservatives labeled Lugar as Obama’s favorite Republican. Mourdock will face Democratic Congressman Joe Donnelly in the November election.

“They want to see someone who is willing to take a stand to challenge the Obama agenda,” Mourdock said earlier on Tuesday while greeting voters at a church in Avon, a suburb of Indianapolis.

Lugar had bemoaned the amount of money being spent on the race by outsiders, most of it on Mourdock.

Super PACs, the unregulated vehicle for a surge in campaign financing this year, poured about $4.6 million into the Indiana Senate primary, according to filings with the Federal Election Commission.

More than $3 million of that total went for ads supporting Mourdock or attacking Lugar, paid for by conservative groups such as Club for Growth, the National Rifle Association and Koch Industries-backed FreedomWorks For America.

While Lugar was backed by popular Indiana Governor Mitch Daniels and Senate colleague John McCain, many conservatives backed Mourdock.

They included former Republican vice presidential nominee Sarah Palin and former presidential candidates Michele Bachmann and Rick Santorum. Most Tea Party groups in Indiana also campaigned for Mourdock.