MARGARET WARNER: We turn to politics now and a veteran senator fighting off a primary challenge in Indiana on May 8.
Gwen Ifill has our report.
SEN. RICHARD LUGAR, R-Ind.: Hi, how are you.
MAN: How are you?
SEN. RICHARD LUGAR: Manning the fort back here.
GWEN IFILL: Behold the endangered species.
SEN. RICHARD LUGAR: Get some more signs back here.
GWEN IFILL: Richard Lugar, at 80 years old, the most senior Republican in the Senate, acknowledged expert on nuclear and foreign policy issues, a fifth-generation Hoosier, is being targeted by his own party for defeat.
Even he seems slightly baffled.
SEN. RICHARD LUGAR: Some persons within our party who say, my way or the highway, they really are less interested, in my judgment, in whether Republicans have a majority in the Senate or the House, than that there be a certain standard in close purity among those who are there.
They do not feel that people ought to work with Democrats across the aisle. Compromise is a bad word.
NARRATOR: Dick Lugar has changed.
GWEN IFILL: The objections come from conservative groups like the National Rifle Association, from super PACs like FreedomWorks for America and the small-government Club for Growth, all crowding the Indiana airwaves with anti-Lugar advertising.
NARRATOR: When Dick Lugar moved to Washington 35 years ago, the national debt was less than a trillion dollars. Now it's over $15 trillion. What's Lugar done
RICHARD MOURDOCK (R), Indiana senatorial candidate: first of all, thank you all for coming out.
GWEN IFILL: Lugar's challenger is state treasurer Richard Mourdock, who was elected for the first time there 2008. And Lugar is now being forced to strike back at one of his own.
NARRATOR: Richard Mourdock's already sold out to D.C. outsider groups, running his campaign on the backs of their money and Mickey Mouse attacks.
RICHARD MOURDOCK: I'm confident there will be a lot of national money flowing in to help us.
NARRATOR: No ideas for Hoosiers, just outsider money and attacks.
MAN: I want to thank God for that, and I want to thank all of you.
GWEN IFILL: Brian Howey, who writes a statewide political letter from his Indianapolis home, has never seen anything like it.
BRIAN HOWEY, Indiana Politics: This is really unprecedented. In the television age of Indiana politics, we have never had an incumbent U.S. senator credibly challenged in a primary. So this particular race is breaking all sorts of molds.
GWEN IFILL: The stakes are high in Indiana, a red state that President Obama turned blue four years ago. Democrats are anxious to solidify their hold on the U.S. Senate, and Tea Party Republicans want to prove to the mainstream GOP that they are here to stay.
Indiana's farm and manufacturing economy faltered badly during the recession. So the state is less friendly territory for Democrats this year. And it's more fertile ground for Republicans like Dwight Lile, who voted for Lugar for years, but will not this time.
And what's changed?
DWIGHT LILE, Mourdock supporter: Well, he's changed. He's a liberal. And I'm not. He doesn't represent the Constitution of the United States the way I think he should. And I can't vote for him. If he were running by himself, I would have to vote for a Democrat, I think.
PAMELA ALTMEYER ALVEY, Lugar supporter: So, you're a winner.
GWEN IFILL: But Lugar loyalists like Pamela Altmeyer Alvey have been moved this year to volunteer for the first time.
PAMELA ALTMEYER ALVEY: Because I think it's desperate. There's such a contrast between the candidates. And I would be appalled to have someone as disingenuous as Mr. Mourdock representing Indiana. He believes there's too much collaboration. It's his ilk that paralyzed the government after the 2010 election.
GWEN IFILL: Mourdock, a Tea Party favorite, says Lugar's Republican vision is dated.
RICHARD MOURDOCK: This is a very broad-based coalition. This is an intraparty rebellion, if you will, against someone who's been there a long time and who unfortunately has separated himself from Indiana Republicans.
RICHARD MOURDOCK: Good morning.
MAN: Nice meeting you.
RICHARD MOURDOCK: Nice meeting you, sir.
GWEN IFILL: Twenty years Lugar's junior, Mourdock says the senator, who has voted for the debt ceiling, the auto bailout and two Obama Supreme Court nominees, is ripe for defeat.
RICHARD MOURDOCK: It is time for a different sense of conservatism in the United States Senate. And I think that is especially true now that Mitt Romney is the nominee-apparent.
CROWD: Mourdock, Mourdock, Mourdock!
GWEN IFILL: Tea Party activist Greg Fettig made up his mind to try to defeat Lugar even before he found another candidate to support.
GREG FETTIG, Hoosier for Conservative Senate: He has drifted to far to the left. And right now a lot of his supporters will say, well, he's a statesman and he works both sides of the aisle. That's fine. But it seems like our side is always -- the conservative side is always asked to come to the center and reach across the aisle, where the left, or the liberals, for example, they don't seem to do that. So we would rather somebody fight for our principles.
GWEN IFILL: But the Tea Party movement is not of one mind. Chuck Ford, another county Tea Party leader, is backing Lugar.
CHUCK FORD, Lugar supporter: The attitude across America is throw the bums out. And I think Senator Lugar has been kind of lumped into that "throw the bums out" phenomena, and inappropriately, but, nevertheless, there's kind of that frustration with Congress that we're not happy with you.
GWEN IFILL: Lugar has also hurt his own cause. Because he sold his Indiana home in the 1970s, he didn't have a legal voting address. What's more, taxpayers were footing the bill for his Indiana hotel bills.
SEN. RICHARD LUGAR: A division of two Democrats and one Republican came to the conclusion that I might be on the ballot running for reelection, but my wife and I could not vote. So that was a shocker, to say the least.
WOMAN: Hello. Thank you for calling Friends of Dick Lugar.
GWEN IFILL: Lugar ultimately changed his legal address to a farm his family owns and repaid nearly $15,000 in hotel costs. But the controversy has helped Mourdock to make his case.
RICHARD MOURDOCK: It is a huge issue, and it is not huge just in the sense of where he is allowed to vote and where he isn't. But it was huge in the sense that he wanted to make a court fight over the fact that you can't make me live in that state. And it was just an incredible thing to watch.
MIKE MCDANIEL, former Indiana Republican Party chairman: That would have been about 1974.
GWEN IFILL: Former state party chairman Mike McDaniel takes heart in recent polls that show Lugar ahead. But he worries that with the primary less than a month away, the margin is not wide enough.
MIKE MCDANIEL: The political environment's changed. One, it has become more of a contact sport than ever. I mean, to claim that somehow this guy is not a Hoosier, it is outrageous. And it's petty politics. And it's kind of the silliness that we're talking about.
This is what politics in America has evolved to, this kind of stuff. We should be talking about a serious issue. He is a serious leader for serious times. We should be grateful for that.
GWEN IFILL: Lugar was not this cycle's only target. Utah's Orrin Hatch spent millions of dollars to diffuse an intraparty challenge in Utah. Now the bulk of the party's anti-incumbency movement is focused on Indiana.
BRIAN HOWEY: One of the mistakes the Lugar organization made was in 2006, when he didn't have a Democratic opponent, which is pretty unprecedented north of the Mason-Dixon Line, I think that they felt like he had ascended firmly into statesmanhood and felt like, if he wanted one final term to finish up his business, there probably wouldn't be a problem. And then, of course, the Tea Party rises up and everything changed.
GWEN IFILL: So the senator who world leaders seek out when they come to Washington is working this spring to repair his base.
WOMAN: Go, Lugar!
SEN. RICHARD LUGAR: There is an implication that we have served long enough, that my age is a factor that would prohibit really active, vigorous service in the future, that it is all well and good to at least salute an elderly gentleman who has done a good job for the state and the country, but, as you say, it's time really to try on something else, something new, something different, something younger.
GWEN IFILL: But you don't agree with that thinking?
SEN. RICHARD LUGAR: No, of course I don't.
SEN. RICHARD LUGAR: I believe that some of the best times are still ahead.
GWEN IFILL: The open question for Lugar is whether those good times will happen in the United States Senate.