Republican Sen. Richard G. Lugar of Indiana, a 35-year member of the Senate and one of Washington’s leading experts on U.S. foreign policy, lost his bid for reelection Tuesday after a conservative backlash inside the GOP denied him his party’s nomination for a seventh term.

Lugar’s loss — the first for a senator this year — appears to be another victory for the tea party conservatives who roiled the Republican Party in 2010 when they defeated two GOP senators in primaries and knocked off several more establishment favorites in open Senate primaries.

With 80 percent of the precincts reporting in Indiana, state Treasurer Richard Mourdock was ahead of Lugar, 60 percent to 40 percent.

In a statement Tuesday night, Lugar said: “If Mr. Mourdock is elected, I want him to be a good Senator. But that will require him to revise his stated goal of bringing more partisanship to Washington. He and I share many positions, but his embrace of an unrelenting partisan mindset is irreconcilable with my philosophy of governance and my experience of what brings results for Hoosiers in the Senate.”

Democrats think Lugar’s primary loss will give them a shot at a seat that has been out of their reach for a long time. Six years ago, Lugar did not even face a Democratic challenger. The 2012 Democratic nominee is Rep. Joe Donnelly, and party members think his reputation as a centrist will stack up well against Mourdock.

Sen. Patty Murray (Wash.), chairman of the Democratic Senatorial Campaign Committee, framed the fall matchup as a battle between “independent” Donnelly and “extreme” Mourdock.

In a fund-raising e-mail sent to supporters after his victory, Mourdock blasted Donnelly’s ties to Obama but, noting Donnelly’s financial edge from having run unopposed in his primary, he pleaded for more finances to wage the general-election battle.

“Joe Donnelly has been a loyal soldier for Barack Obama. He was a deciding vote on ObamaCare. He voted for Obama’s Stimulus program and his bailouts. Donnelly voted for Obama’s policies so often that the President calls him his partner,” Mourdock wrote.

Republicans need four seats to take control of the Senate, and the path to that majority would be considerably more difficult if they lost Lugar’s seat.

But Republicans contend that Indiana is a solidly Republican state and that, even though Barack Obama in 2008 became the first Democratic presidential candidate to win it since 1964, that Republican advantage will give them the edge they need.

The National Republican Senatorial Committee circulated memos Tuesday evening touting Donnelly’s 2010 vote for President Obama’s health-care law, which remains unpopular in Indiana.

Republican insiders say Lugar’s loss was probably more the result of several years of self-inflicted wounds and less about the strength of the tea party in Indiana. Lugar faced questions about whether, after decades of bipartisan dealmaking in Washington, he had lost touch with his state’s voters.

GOP senators and national party strategists think that Lugar, 80, who was first elected in 1976, ignored their advice about how to run a more vigorous and effective campaign, thinking he knew his state better than they did, even though his last tough reelection bid was 30 years ago.

Before the returns were in, Republicans said lessons could be learned from Lugar’s race. “The moral of the story is: Don’t play defense, play offense — one of the fundamental rules of elections,” said Sen. John McCain (Ariz.), who endorsed Lugar.

McCain, despite being the GOP’s presidential nominee in 2008, faced a potentially difficult reelection in 2010, but he ran one of the the most aggressive primary campaigns that year of any lawmaker seeking another term. He spent more than $20 million to beat his opponent, winning by 25 percentage points and cruising to victory in the fall.

But Lugar is more the diplomat and has none of McCain’s warrior profile. He had no history of the aggressive campaigning that was being asked of him. Lugar, the ranking Republican on the Foreign Relations Committee, is a soft-spoken and deliberate man who often seems more comfortable with briefing books on nuclear proliferation, on which he was the Senate’s leading expert.

The result was that he was something of an anachronistic figure in the current era of permanent, scorched-earth campaigns. In July 2008, after Obama had locked up the Democratic nomination, he aired ads touting his work with Lugar on the foreign relations panel. Rather than protest in an effort to help his party’s nominee, Lugar told reporters how happy he was to have worked closely with Obama to pass legislation dealing with the post-Soviet-era nuclear stockpile.

“So I am pleased we had that opportunity to work together,” Lugar said, even though he was supporting McCain.

The White House issued a statement from the president about Lugar’s loss. “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,” Obama said. “ . . .He has served his constituents and his country well, and I wish him all the best in his future endeavors.”

Vice President Biden, one of Lugar’s longtime counterparts on the Foreign Relations Committee, phoned the senator to thank him for their partnership. “We never had a cross word. In matters of foreign policy, we seldom disagreed,” Biden, who was the ranking Democrat on the panel for 12 years, said in a statement given to The Washington Post.

Conservatives have long questioned how committed Lugar was to their causes, dating to 1986, when Sen. Jesse Helms ousted Lugar as the top Republican on the foreign relations panel after the midterm elections that year. Helms portrayed Lugar as an internationalist who sided with the United Nations. In 1995, Lugar used his foreign relations expertise to seek the GOP presidential nomination, but the bid never got off the ground. He announced his candidacy the same day as the Oklahoma City bombings.

When it was time to seek a seventh term, Lugar’s transgressions against conservative orthodoxy extended far beyond his foreign policy views. He voted for Obama's Supreme Court nominees, he supported a comprehensive overhaul of immigration law in 2006, and he backed a scaled-down version of that legislation in 2010 that created a pathway to citizenship for children who came to the United States illegally with their parents.

Outside conservative groups poured millions of dollars into this year’s primary to bash Lugar or support Mourdock. But Lugar’s final undoing may have had more to do with his own mistakes than any tea party-related activity, senators and strategists said.

After taking office in 1977, Lugar sold his house in Indianapolis and settled in the Washington region, where he has lived for the past 35 years while continuing to vote in Indiana using his 1970s address. He stumbled in interviews with local media outlets trying to explain how he could live full time in McLean.

Republicans said the next few Senate primaries will provide a clearer picture of the tea party movement’s strength nationwide.

In Nebraska, state Attorney General Jon Bruning has the support of most senior state leaders for the GOP nomination for the seat of retiring Sen. Ben Nelson (D), but tea party activists are backing former attorney general Don Stenberg. In Texas, Lt. Gov. David Dewhurst has the most establishment support in the GOP primary battle to succeed retiring Sen. Kay Bailey Hutchison (R), but state Solicitor General Ted Cruz is trying to rally conservatives.