Never!
If you believe that the worse thing that could possibly happen is 4 more years of Barry the great pretender .. then the logical thing to do is cast your vote to ensure that the “worse” can’t happen. Since I condsider the economy and jobs to be the overriding priority, then for me, that means I vote for someone I believe is “electable” and will address the priority. While I respect your right to cut off you nose to spite your face, I will vote for Romney, or Gingrich as one of these are likely to be the nominee. Unless the third party candidate is Al Gore, then all a third party does is give Oblamer another 4 years. Quite frankly, we can’t survive 4 more years.