Tuesday's primary vote in Indiana pits Senator Lugar against state Treasurer Richard Mourdock, whose challenge is rooted in the themes that Lugar is out of touch both with his state and with an increasingly conservative Republican Party.
Mr. Mourdock's insurgency has shifted in recent weeks from a long-shot bid to an odds-on favorite as Hoosiers head to voting stations.
But with Lugar seeking to snatch victory from the jaws of defeat, here's one thing that the six-term Senate veteran can be glad of: Rick Santorum is no longer competing against Mitt Romney for the Republican nomination.
The primary outcome hinges on what types of voters turn out, and a hot presidential contest involving former Senator Santorum of Pennsylvania would have drawn more hard-core conservatives, including evangelical Christians, says Brian Vargus, a professor of political science at Indiana University in Indianapolis.
"If there was still a fight for the nomination between Santorum and [Mitt] Romney, Lugar would be toast," Mr. Vargus says. He notes that Santorum has endorsed Mourdock's bid to replace Lugar as the state's Republican Senate candidate.
In contrast to the live presidential battle that was being waged a few weeks ago, Tuesday's Indiana primary coincides with a move by Santorum to officially endorse Romney. The former Massachusetts governor, for his part, held a fundraiser in Indiana Monday.
Still, Mourdock appeared to have the momentum heading into voting day. A Howey/DePauw poll of 700 likely voters, released last week, showed Mourdock leading by 10 percentage points.
Mourdock is banking on voters who are in a mood of tea party rebellion. Lugar's best hope is that lots of moderate and independent voters show up for the open primary vote.
Lugar has been a US senator since January 1977, making him the longest-serving Republican along with Orrin Hatch of Utah, who arrived in the same Senate class. On the Democratic side, Patrick Leahy of Vermont has served two years longer, and Daniel Inouye of Hawaii took office in 1963.
Mourdock's ads have sought to turn Lugar's age and long tenure into a liability, talking about the senator moving to Washington at a time when disco and leisure suits were in fashion. The ads have also labeled Lugar "Obama's favorite Republican."
Lugar's voting record puts him in the dwindling ranks of moderate Republicans, with a conservative voting score of 65 percent over his Senate career, according to the Club for Growth. In the conservative group's ratings, many Republican senators post scores in the 80s or 90s, and Lugar himself moved from a score of 70 in 2010 to 80 last year.