In the letter, Lugar Political Director David Willkie encouraged Indiana Republicans not to follow the lead of the Delaware, Nevada and Colorado GOP, arguing that if they had taken "the Reagan 'big tent' approach [in 2010], we would have already repealed Obamacare.
"I encourage all Republican leaders to rally around the Dick Lugar flag now; you have been led down a wrong path and it's time to come home," Willkie wrote, referencing the entry of Rep. Joe Donnelly (D-Ind.) into the Senate race Monday.
Democrats see an opening in 2012 should Mourdock, who has won the backing of several Tea Party groups, defeat Lugar in a GOP primary. The Democratic Senatorial Campaign Committee labeled it a prime pickup opportunity with Donnelly in the race.
Still, Mourdock said Tuesday that he's confident he can beat the Democrat should the two face off in next year's general election, comparing Donnelly's profile to that of former Rep. Brad Ellsworth (D-Ind.) who lost a Senate race to Republican Dan Coats last fall.
"I'm sure Donnelly will try to run as a Blue Dog, pro-life, pro-gun Democrat," Mourdock said. "That's the same thing Brad Ellsworth did last time and I'm not sure what he's doing now because he was soundly defeated."