INDIANAPOLIS — Of all the words written about the showdown between U.S. Sen. Richard Lugar and State Treasurer Richard Mourdock, this headline from a recent news story may sum it up best: “Indiana’s GOP Senate Primary Will Be A Doozy.”

That’s not a headline the six-term Lugar and his supporters expected a year ago. At 80, the longest serving Republican in the U.S. Senate hasn’t had a primary challenger since 1976 — the year Gerald Ford narrowly defeated Ronald Reagan in the GOP presidential primary.

It is the kind of headline Mourdock, 60, and his backers have been predicting. When he announced his decision last year to launch an intra-party challenge, Mourdock cited results of a survey that found  a majority of Republican primary voters were willing to consider voting for someone other than Lugar.

Mourdock has spent the months since then explaining why GOP voters should replace a man who’s been venerated as an elder statesman. Lugar, in his influential role on the Senate Foreign Relations Committee, long ago landed a place in the history books. After the Cold War ended, he helped negotiate a bipartisan plan to divest the former Soviet Union of its nuclear and chemical weapons. He forged a similar plan in 2005, with then-Sen. Barack Obama, to provide assistance to countries that wanted to rid themselves of weapons of mass destruction.

“Hoosiers want more than a globe-trotting senator,” Mourdock declared when he announced his candidacy.

Later, he told the Wall Street Journal: “We’re convinced that Republican voters in Indiana want a more conservative voice.”

Mourdock, in his second term as state treasurer, has been working to convince primary voters he’s that voice. He argues the federal government has veered far from the Founding Father’s original intent; in 2009, in his role overseeing Indiana pension funds which had a stake in Chrysler, he opposed the federal government’s bailout of the bankrupt auto maker as unconstitutional.

Mourdock has garnered money and support from Tea Party groups in and outside of Indiana, along with the anti-tax Club for Growth and the National Rifle Association.

As of mid-April, Mourdock was polling within a few points of Lugar, but Lugar had nearly a 6-to-1 campaign cash advantage over Mourdock. Lugar also had the endorsements of the U.S. Chamber of Commerce and the public backing of Indiana Gov. Mitch Daniels and Arizona Sen. John McCain, both Republicans who’ve made campaign ads on Lugar’s behalf.

On Friday, Mourdock received an endorsement from former GOP vice-presidential hopeful Sarah Palin.

In early April, Lugar and Mourdock met for their sole debate and engaged in an hour-long exchange most noted for its civility. On the campaign trail and interviews with national media outlets that have turned their attention to the race, their differences are more pronounced. The New York Times quoted Mourdock denouncing Lugar’s reputation as a bipartisan solution-seeker: “The time for being collegial is past,” he said. “It’s time for confrontation.”

In a post-debate interview with PBS NewsHour, Lugar dismissed his critics as unyielding purists, “who say, my way or the highway ... They do not feel that people ought to work with Democrats across the aisle. Compromise is a bad word.”

Longtime political observer Brian Howey, publisher of the Howey Indiana Politics newsletter, said the race will be doozy to the very end.

“In the television age of Indiana politics, it’s unprecedented,” he said. “We’ve never had an incumbent U.S. senator credibly challenged in a primary. So this particular race is breaking all sorts of molds.”

The winner of the May 8 GOP primary meets U.S. Rep. Joe Donnelly, a South Bend Democrat elected to Congress in  2006.