The last big Republican primary battle of 2010 looks to be in the "First State" of Delaware, where the primary will be held on September 14th.

The seat, formerly occupied by Vice President Biden, is a coveted GOP pickup opportunity this fall.

The frontrunner and establishment candidate is Delaware's lone Congressman and former Governor, Mike Castle, who has served nine terms in the House.

But with 13 days left the Tea Party Express PAC is getting involved - the same group that helped lift Joe Miller to victory in Alaska's GOP Senate primary over Sen. Lisa Murkowski.The Tea Party Express is backing conservative activist Christine O'Donnell over Castle, and the Delaware GOP is determined not to make the mistake of ignoring the threat. Responding aggressively to O'Donnell and her backers, Delaware State GOP Chairman Tom Ross made his thoughts on the race very clear - click below to watch.

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Carl Cameron currently serves as Fox News Channel's (FNC) Washington-based chief political correspondent. He joined FNC in 1996 as a correspondent.