Down, Down, Down?

Casey's going down, down, down, in a burning ring of fire...right? According to a survey of 578 registered voters, Casey's lead over Santorum has dwindled to 6%. Casey leads 47-41.

However, this survey oversamples Republicans. According to Santorum Blog:

The poll included 46.8% Rs, 42% Ds 9% Independants. State wide the registration numbers are 47.6% D, 40.8 R, 11.6% other. Democrats were underpolled and Republicans were overpolled.

Despite this statistical tomfoolery, Casey's lead has been collapsing over the past month or two. Maybe Chuck Pennacchio was right to criticize the methodology of Terry Madonna and Keystone Polling?

As an aside, remember when Casey had a double digit lead over Rendell less than one month before the 2002 Gubernatorial race only to lose in the primary election? Does anyone else see pattern behavior here?