Sen. Dick Lugar (R) has been a conservative target for over a year. But only for the past month has the senator started to fight back against state Treasurer Richard Mourdock. With the primary coming up on May 8, it’s likely too little too late.

A yard sign calling for voters to "retire" Sen. Richard Lugar stands along the road in New Palestine, Ind., Friday, March 30, 2012. (Michael Conroy/AP)

Lugar’s allies no longer seem interested in beating up the man who could very well be their nominee come November.

The American Action Network, a Republican-backing outside group*, is pulling its ads in support of Lugar off the air Tuesday. “We’ve decided we’re going to let this race play out,” said spokesman Dan Conston.

The Young Guns Action Fund, a super PAC affiliated with House Majority Leader Eric Cantor (R-Va.), is still supporting Lugar — with mailers reminding Democrats and independents that Indiana has an open primary.

“You have to drop mail a long time before it comes out,” said YGAF founder John Murray. “I would not expect that we’re going to be doing a whole lot more.”

The Club for Growth, meanwhile, has spent about $945,000 attacking Lugar and another $500,000 supporting Mourdock.

But Lugar had millions of dollars more than Mourdock in the bank. He could have used his overwhelming financial edge to define Mourdock months ago, the way Sen. John McCain (R-Ariz.) did former Rep. J.D. Hayworth in the 2010 primary while shoring up his own conservative bonafides.

The old conventional wisdom for incumbents — don’t punch down and elevate your opponent — no longer applies.

“Campaigns matter. You've got to stop [a primary opponent] before the genie gets out of the bottle, or you're in real trouble," said one national Republican strategist. “The only way to prevent them from getting money is to knock them out right away."

While Lugar has gone negative, it was only in response to Mourdock’s attacks. Recent attempts to land a solid hit — for example the “critical” revelation that Mourdock supported the Fairness Doctrine in 1992 — have fallen flat.

A tough primary for Lugar was almost inevitable.

He’s been in the Senate 36 years. He’s a centrist Republican, a dying breed — especially in an increasingly red state like Indiana. He backed the Wall Street bailout, the auto industry bailout, gun control legislation, the DREAM Act, and numerous other bipartisan initiatives.

Leaders of nearly 70 tea party groups got together in February of 2011 to declare their opposition to the veteran senator. Mourdock got into the race shortly thereafter.

Mourdock, on the other hand, represents the current mood of the party when he says things like this: “We don't need bipartisanship as much we need as the application of principle.”

While he’s shifted some in response to Mourdock’s challenge — for example, dropping his co-sponsorship of the DREAM Act — Lugar also taunted his tea party antagonists, telling the movement in 2011 to “get real.”

Lugar’s campaign argues that obituaries are premature.

”We have a campaign plan that was developed two years ago in terms of a microtargeting effort, and that is continuing to be executed as planned. We feel very confident,” said Lugar campaign spokesman Andy Fisher. He said early voters are breaking for the senator.

An independent poll hasn’t been taken in the state since a month ago, when Lugar led by seven points. A poll for the Mourdock-backing group Citizens United last week had Mourdock ahead five. There’s a chance Lugar could pull this out. But if he doesn’t, it wasn’t because he didn’t have enough warning.

* AAN has an affiliated super PAC but is a 501 c4; this post has been corrected.