RedState reported the plan many months back.
Here it is.
http://www.redstate.com/erick/2010/08/02/why-is-mike-castle-running-from-christine-odonnell/
Posted by Erick Erickson
Monday, August 2nd at 8:00AM EDT
192 Comments
If Mike Castle becomes the next United States Senator from Delaware he is going to get sworn in, serve a bit, then become a Democrat, resign, and let Beau Biden get an appointment.
It’s a done deal. The deal has been made already. I’m hearing this not just from Christine O’Donnell people, but party regulars in Delaware, some of whom support Castle. It is insane.
Maybe that is why Mike Castle is running from Christine O’Donnell. He is afraid to have to answer questions about this. The man will not debate her to save his life.
(snip)