Basically, as a liberal, he seemed to make common cause with the Democrats (and was probably quite happy to switch places with Tom Carper when the latter succeeded him as Governor, excluding a brief seat-warmer). He did next to nothing to bolster the standing of the GOP and was pretty clearly disinterested in trying to displace either Carper or Biden (let alone the succession of Dems in the Governorship ever since). I also suspected that his Senate “run” in 2010 was going to see him support the Democrat agenda in exchange for covert support (Coons, I alleged, to be a ringer for him), switch parties, and step down to allow Biden’s son to succeed him in the Senate. O’Donnell threw a wrench into those plans.
Given the number of senators who have expired in office lately (Inouye, Kennedy, Byrd, Launtenberg) or might have (Specter had he won another term, perhaps Lugar if he hadn’t been sacked in the primary, and Orrin Hatch and John McCain seem to be going for this, too), I think Castle might have been another lifer. The fact that he would run for Senate at 71 and after suffering two minor strokes back in 06 suggested he didn’t care he was way past his prime (not unlike Tommy Thompson in WI), he just saw what looked like an easy route to the Senate and took it. Odds are he’d have served until he came out with a toe tag.