Beware your presumptions. I don't know too many who are planning to sit it out. We will definitely vote down-ticket, and many will vote 3rd party for the presidency.
My primary reason for voting will be to replace Debbie Stabenow. As far as Romney is concerned, his chances are worse than John McCain’s were.
If the nose holders really want to take the white house, their best bet is to join us and force the GOP to fix this even if it means forcing Romney out.