well, that sucks. And Delaware voters will probably be stupid enough to vote for him, thinking he's a "moderate", despite the fact that he'll vote to make far-right Republicans heads of the various Senate committees.
So by this logic there hasn't been a "moderate" of either party in the Senate for decades.
They can be moderate, it's just that their moderation doesn't do very much when, for example, they vote for Jim Inhofe to chair the Environment and Public Works Committee or Jeff Sessions to chair the Judiciary Committee or Mitch McConnell to decide which bills and amendments are voted on. I don't deny that Castle might vote with Democrats on a couple of bills (though I doubt he'd support cap and trade or robust health care reform), but just because he's a moderate doesn't mean that he's not making some very, very conservative elements, elements that the voters in Delaware do not want running the country, more powerful.