Club for Growth is a PAC that generally supports candidates in favor of low taxes, free markets, and supply-side economics.
They are known for running ads against more liberal Republicans like Arlen Specter and Lincoln Chafee. They were a big force behind the Pat Toomey candidacy in Pennsylvania.
Some may not like them as they support conservative candidates other than the party pick in some primaries and have been successful. They backed Senator Coburn and I think also Representative Pence.
Club for Growth = Open Borders Lobby.
The Club for Growth supports Bill Sali in that Idaho race because he is the only one of the six republicans stupid enough to support CAFTA.