Not that Colin Powell's been known to read this blog, but I would say to him that if he wants the tradition of center-right Republicanism to remain alive, and have a chance at thriving, then a speculated-upon endorsement of Obama would not be the way to do it. If McCain loses, and loses big, which becomes yet more plausible if Powell backs the Democrat, then the never-compromise, never-rethink-anything element of the Republican Party will become more dominant within the party than it ever has been. That also means the party will have a decent chance of never winning a presidential election again, and going the way of the Federalists.
If, by the way, Powell surprises the pundits by endorsing McCain, then we have a whole different story. Whether McCain wins or not--and his chances of winning would significantly increase--being backed by Powell would be a message that the Republican Party genuinely is, as it used to be called, a "big tent" with a conservative core but room for a healthy range of political views. This would also help prevent a stultifying dominance by a Democratic Party in which the Clintons are now suspiciously centrist. Just a thought.
UPDATE 10/19, 10:40 AM: Sadly, I couldn't persuade him.