Longtime U.S. Sen. Richard Lugar appears to be shifting his re-election message to focus on attacking national interest groups, which the Republican accuses of having an exaggerated say in his Indiana race.

A new statewide television ad that began airing this week alleges that Lugar's Republican challenger, Indiana Treasurer Richard Mourdock, has "already sold out to DC outsiders" because of how much he's leaning on spending by groups outside the state.

Lugar's opponents, however, have spent hundreds of thousands of dollars hammering a storyline that it's the senator who is the Washington outsider, noting that he lives fulltime in a Washington suburb and has spent 35 years in the Senate.

Lugar and Mourdock are set to meet Wednesday in the lone debate before the May 8 Republican primary.

The anti-tax Club For Growth opened its third round of attacks ads Monday against Lugar, and the National Rifle Association is sending more than a million mailers to its Indiana members in its first foray since endorsing Mourdock last month.

Until this week, the Lugar team has spent most of its money attacking Mourdock for his attendance at state boards, alleging that he doesn't personally attend enough meetings, and attacking President Barack Obama for blocking construction of the Keystone XL oil pipeline out of Canada.

Now, after a trio of recent polls showed Mourdock closing in on Lugar, the senator's team is focusing the campaign ads on outside interests supporting Mourdock.

"It's obviously been a concern that has come up with voters that we have talked to," Lugar spokesman Andy Fisher said. "People don't like to have out-of-state political operatives pushing decisions on them."

Chris Cox, executive director of the NRA Institute for Legislative Action, noted that the senator never seemed have a problem accepting such endorsements in the past, including one in 2006 from the Washington-based Brady Center to Prevent Gun Violence.

"It's not an outside interest as long as it supported a gun control agenda," Cox said. "It's so hypocritical it becomes laughable."

Local Democrats, along with a loose group of conservative interest groups, hit on a storyline earlier this year that seemed to strike gold: accusing the longtime senator of being out of touch with Indiana residents because he hasn't owned a home in the state since 1977.

That constant attack, combined with a Marion County elections board ruling last month that threatened Lugar's eligibility to vote in Indiana, gave the odd political bedfellows a united front against Lugar.

As of last week, a trio of outside groups supporting Lugar had bought $370,000 worth of airtime across the state, while the Club For Growth alone had bought roughly $735,000 to oppose to Lugar.

Super PACs have sprouted up as a potential force in the Senate race, with two forming to back Lugar, and Lugar opponents splitting their spending between the parent group and the PAC.

By far the biggest spender in the air wars, however, has been Lugar himself, who has bought roughly $1.9 million of airtime. Mourdock has paid for $360,000 of airtime, according to spending totals maintained by Indiana Democrats.