Outside interest groups have poured millions of dollars into Indiana before Tuesday's Republican Senate primary.

As Tea Party-backed state Treasurer Richard Mourdock seeks to uproot incumbent Dick Lugar, who ties Utah's Orrin Hatch for the longest tenure among any GOP senator, conservatives see a chance to remove one of the last few centrists in the upper chamber. Polling now shows Mourdock in the lead.

Rare for a primary, Indiana has seen the candidates themselves spend at a breakneck pace. As of their latest disclosures on April 18, Lugar had spent $6.7 million defending himself against Moudock, who had spent $2 million himself.

A total of 11 groups have spent money on the race. Only one among them, the USA Super PAC, was formed in Indiana. Organized in April, it does not appear to have a website. It made its first and only expenditure, a $97,500 media buy supporting Mourdock, on Thursday.

Two more super PACs, Indiana Values Super PAC and Hoosiers for Economic Growth and Jobs, formed this year in Washington, D.C., and Sacramento, Calif., respectively, both for the purpose of attacking Mourdock.

In a potential sign of trouble for Lugar, the Associated Press this week reported that one of his biggest supporters, the D.C.-based 501(c)4 group American Action Network, had canceled plans to spend an additional $200,000.

Interest groups have spent more than $4.6 million on the primary, and the pace has sped up significantly. As of April 25, eight outside groups had spent $2.2 million. In the last 10 days, three more groups have gotten involved, and groups have collectively spent $2.4 million more.

Seven groups have spent a total of $2.9 million in support of Mourdock:

Club for Growth - $1,675,221.11

FreedomWorks - $538,495.77

NRA - $491,066

USA Super PAC - $97,491.53

Citizens United - $96,300.00

Majority PAC - $23,500

Tea Party Express - $6,431.95

Four have spent $1.7 million in support of Lugar:

American Action Network - $676,548.10

Indiana Values Super PAC - $581,694.00

Hoosiers for Jobs - $214,590.00

Young Guns Network - $241,078.00