WILMINGTON, Del. -- For supporters of Rep. Mike Castle (R-Del.), all eyes will be on Delaware's New Castle County Tuesday where the level of turnout in the state's contested GOP Senate primary could make or break the longtime congressman.

New Castle County houses the urban population centers of Delaware. It contains Newark and Wilmington, the state's largest city. 

The Castle campaign is hoping for solid turnout Tuesday in and around Wilmington to help the centrist congressman offset more conservative primary-goers in the southern part of the state who may be backing Tea Party favorite Christine O'Donnell. 

O'Donnell is banking on support from an energized conservative base to help her to an upset Tuesday. 

The best estimates of observers in the state put turnout in Tuesday's closed Republican primary around 40,000 voters. For Tea Party-backed Christine O'Donnell, the lower the turnout, the better on Tuesday. 

At least 70 Castle volunteers were campaigning door-to-door Sunday, many aimed at drumming up turnout among Castle-friendly Republicans in the northern part of the state.    

Near Wilmington, it's not tough to see that the area is Castle's base. Castle for Senate billboards and signs are numerous in Wilmington and just outside of the downtown area, while O'Donnell signs are hard to come by.

At least one small O'Donnell for Senate sign was spotted alongside Route 202, close to downtown Wilmington. It stood in the shadow of a much larger Castle sign and a bright red campaign sign for Glen Urquhart, the conservative challenger running in the primary for Delaware's at-large House seat. 

O'Donnell is banking on turnout from the state's two counties just to the South of New Castle--Kent and Sussex counties.

The Tea Party Express has a phone bank set up in-state to make GOTV calls on O'Donnell's behalf and the group is coordinating an effort with supporters from out of state to phone Delaware voters ahead of Tuesday.