Poli-ticks

A friend of mine was saying that Obama should/would run as a third party candidate and take on a Republican vice-president if Clinton got the nomination.

At first, I thought: great! But it would be a totally double-edged sword. He would take more Democratic votes away from her than he would take Republican votes from (presumably) McCain.

Do you think he would prevail, or would this definitely be another Nader-case?