Indiana's Senate race is shaping up to be an expensive battle among outside interests, and that's only the Republican primary.

At the beginning of April,  Sen. Richard Lugar, the longest-serving GOP senator (tied with Utah's Orrin Hatch) had spent nearly $5.1 million fending off a primary challenge from state treasurer Richard Mourdock, a Tea-Party-backed conservative who had plopped $1.7 million of his campaign war chest into the race.

Despite the competitive and potentially bruising primary, which takes place May 8, its victor is expected to beat Democrat Joe Donnelly, a third-term congressman from northern Indiana.

But perhaps the most intriguing aspect of the Republican race is the panoply of interest groups that have gotten involved, a rarity in primaries, much less primaries in which there's a long-tenured incumbent.

The Washington-D.C.-based 501(c)4 group American Action Network rolled out its second ad attacking Mourdock  Wednesday, this one accusing him of taking an "illegal tax break" (Mourdock has denied this and called for AAN to pull previous ads making similar allegations), and that he recklessly mishandled state money.

It was just the latest instance of out-of-state groups pouring money into the race, as nearly $2.2 million has  been spent by a total of eight groups formed outside Indiana. The race has pitted the traditional set of D.C.-based conservative groups, which cherry pick a few vulnerable GOP moderates every election cycle, against a small collage of the establishment and new pro-Lugar super PACs, which have come to the long-time senator's defense.

Groups supporting Mourdock have spent a total of nearly $1.5 million, while a handful of groups have come to Lugar's aid, spending nearly $800,000 in response.

Lined up behind  challenger Mourdock, predictably, are FreedomWorks and the Club for Growth, two Washington-based groups that make a practice of supporting fiscally conservative challengers in Republican primaries. FreedomWorks has spent nearly $300,000, almost all of it on polling, online ads attacking Lugar, paid- organizing staff  and signs. The Club for Growth, typically preferring the air wars, has spent more than $850,000 flooding the state with TV ads, direct mail and radio advertising attacking Lugar's record and promoting Mourdock as the conservative choice.

Also backing Mourdock is the National Rifle Association, which gives Lugar an  F  grade and endorsed his challenger in March. The group has spent over $320,000 on phone-banking, TV, radio, and mail. The group has borne a grudge against Lugar since he voted for the Brady Bill in 1993, as The Atlantic Wire has pointed out. And it's perhaps not the backing Mourdock wants, but Majority PAC, the Democratic super PAC dedicated to Senate races, reported spending $10,000 on Internet ads attacking Lugar.

The American Action Network, founded by former Minnesota Sen. Norm Coleman  and all-around GOP activist and fundraiser Fred Malek, sprung to Lugar's defense, pledging nearly $600,000 worth of ads earlier this month. So far, the group has spent nearly $344,000 of that.

The group's communications director, Dan Conston, reasoned on Wednesday's ad announcement that electing Mourdock would "only embolden" Democrats in November.

YG Network, a Virginia-based 501(c)4 group formed by former aides to House Majority Leader Eric Cantor and Majority Whip Kevin McCarthy, weighed in on Wednesday with a $104,000 mailer supporting Lugar. A top official with the group said the spending was part of a larger energy-issue campaign involving other candidates.

Also backing Lugar are two super PACs that formed this year: Indiana Values, organized in Washington, D.C., in January with the sole mission of backing Lugar's re-election bid, and in March it began spending nearly $137,000 on polling, online  and TV ads. Hoosiers for Jobs formed in early April in Sacramento, Calif., also for the lone purpose of backing Lugar, and has spent nearly $215,000 airing ads that don't attack Mourdock as much as they attack the Club for Growth.

While all this mud is getting slung on the airwaves, the candidates  have  conducted themselves in notably mild manner: The one time they shared a stage, at the only scheduled primary debate, the two barely attacked each other. For such a contentious primary, the biggest surprise was that they seemed to agree on almost everything.