Castle is a Democrat in everything but name. We aren't talking about a person who agrees with us "70-80% of the time". We are talking about a 70 year old guy who votes more like John Kerry then Jim Demint.
I don't think that is true, from what I have been able to read. However, even if he voted like a Dem, he could allow the GOP to control the Senate. The alternative is to elect a Dem, who will vote like John Kerry and have Harry Reid or Dick Durbin lead the Senate.
(I didn't bother addressing the possibility of O'Donnell beating Coons - I don't deal in fantasy.)