Sen. Richard Lugar, R-Ind., lost a primary battle Tuesday to tea party challenger and Indiana state treasurer Richard Mourdock for the seat he has held since 1977.
Lugar's defeat makes him the first and likely only senator to lose renomination this year.
After returns from the primary came in, Lugar conceded to Mourdock before a crowd of supporters. He says he wants to see a Republican in the White House and will support Mourdock in his race against Democrat Joe Donnelly for the Senate seat.
Mourdock's supporters cast Lugar as too moderate and out of touch after 35 years in the Senate. The American Conservative Union gave Lugar a 77 percent "lifetime" rating. Lugar's supporters claimed it hardly constituted a moderate voting record. But tea party conservatives argued Mourdock would provide more "purity" and wouldn't bow to compromise with Democrats.
Sens. Orrin Hatch, R-Utah, and John McCain, R-Ariz., avoided situations similar to Lugar's by moving further to the right ideologically. Lugar, however, initially refused to do the same. He didn't run a slew of negative advertisements against his opponent until late in the primary campaign. Even then, some critics said they stood in contrast to his reputation as a statesman.
Lugar's campaign spent $6.7 million compared to Mourdock's $2 million. But Mourdock's political action committees spent $2.9 million attacking Lugar over the incumbent backers' $1.7 million.
One advertisement from Mourdock's campaign called Lugar "President Obama's favorite Republican." It showed clips of President Obama saying, "I've worked with Republican Senator Dick Lugar to pass a law" and "What I did was reach out to Senator Dick Lugar."
Lugar, 80, built a Senate career largely focused on foreign policy. The two-time chairman of the Senate Foreign Relations Committee became a leading voice on nuclear weapons. His signature achievement was the 1991 Nunn-Lugar Act, which he wrote with former Sen. Sam Nunn, D-Ga.
Lugar didn't help his campaign when it came to light that he no longer owned a residence in Indiana. He instead stayed in an Indianapolis hotel when he returned to the state – using taxpayer funds. Mourdock's campaign said the revelation symbolized how Lugar had become too entrenched in Washington.
The senator said that his office had billed taxpayers for the hotel expenses due to staff errors. In late March, Lugar reimbursed the Treasury $4,500 for hotel expenses over the last decade.
Mourdock now faces Rep. Joe Donnelly, D-Ind., in the general election. Political observers say Donnelly's moderate voting record could appeal to independents and even some Republicans who don’t like Mourdock's conservative platform.
Lugar supporters argued that a Mourdock victory could increase Democrats' chances of winning the Senate seat in a reprisal of far-right candidates in 2010, such as Christine O'Donnell in Delaware and Sharron Angle in Nevada, who lost against Democratic opponents.
After conceding to Mourdock, Lugar congratulated Mourdock and said he "hopes his opponent prevails in November."
Republicans need four seats to win the Senate majority.
The Associated Press contributed to this report.