The Tea Party Express endorsed Mourdock's Senate bid Thursday. Executive Director Amy Kremer said Lugar's voting record has shifted "increasingly to the left" and that he needs to be replaced.
The group already announced plans to target Lugar for defeat in 2012.
The tea party group started by veteran California Republican operatives spent heavily in the 2010 elections with mixed results.
While backing unsuccessful candidates for Senate in Alaska, Delaware and Nevada, the group also supported Florida Sen. Marco Rubio's successful bid and bolstered Republican Scott Brown's upset in Massachusetts.
Mourdock picked up a separate endorsement from Indiana tea partiers last weekend.