The air wars have officially commenced in the Indiana Senate race, with the National Rifle Association and Club for Growth launching new ads against Sen. Richard Lugar (R-Ind.) and Lugar responding in kind with an ad attacking opponent Richard Mourdock.

A yard signs calling for voters to "retire" Sen. Richard Lugar stands along the road in New Palestine, Ind., Friday, March 30, 2012. Lugar, running for his seventh term in the U.S. Senate, is facing one of his toughest election battles in the Republican primary against state Treasurer Richard Mourdock. (AP Photo/Michael Conroy)

The NRA ad touts Lugar’s ‘F’ rating on the group’s lawmaker scorecard, while the Club for Growth ad keeps up an offensive focused on Lugar’s votes on bailouts, tax hikes and President Obama’s Supreme Court justices. Both note the three-plus decades Lugar has spent in the Senate.

“Over 36 years in Washington, Dick Lugar has changed. He’s become the only Republican candidate in Indiana with an ‘F’ rating from the NRA,” the NRA ad says. “It’s time for another change.”

Lugar’s ad, meanwhile, fights back by painting Mourdock, the Indiana state treasurer, as a tool of Washington special interests who are bankrolling his campaign.

The ad features audio of Mourdock saying he’s “confident there will be a lot of national money flowing in to help us.” The ad then shows Indiana residents saying unkind things about Mourdock and his political ambition.

Lugar’s campaign had $4 million in the bank at last check, which should be enough for an extensive presence on television in the runup to the May 8 primary.

Mourdock, as of the end of 2011, hadn’t been raising big money, but groups like the NRA and Club for Growth promise to spend plenty to level the playing field.

A bipartisan poll released last week in Indiana showed Lugar leading Mourdock 42 percent to 35 percent, further indication that Lugar is the senator most vulnerable to a primary challenge this year.