In Obama's case, I'm not talking about whether the candidate gets into office or not (that's out of any strategist's hands), but what's going to happen to the engaged, interested, willing-to-work/give community he's developed via MyBarackObama.com.
My advice: Keep it going.
These folks, many of whom are campaign/advocacy virgins, are enthused. They've been awakened, or re-awakened, by a passion for change, and by a candidate who's made it possible for them to grow into a community of like-thinkers. And they're well-positioned to do more; for other candidates, for the party, for key issues.
Jesse Helms, a mastermind marketer, transitioned campaign supporters' contact info (phone and address at that point) into what then became a very powerful National Republican Congressional Committee.
Obama, and your organization — at the close of an advocacy or issue campaign — should do the same (not to the NRCC, but to an ongoing community of interest and action around whatever they've responded to initially).