Ralph Nader just hates Democrats more than anything, including Republicans. It would be one thing if Nader was advancing some theory that running left-wing spoiler campaigns in the general election will hurt Democrats but push the Party to the left. That's a terrible theory, but at least it's a theory. But it's pretty clear that Nader isn't even advocating this kind of crazy long-term plan when you consider things like his advocacy of a Mike Bloomberg presidential campaign:
“In area after area, you know, whether it’s consumer fairness, single payer health insurance, full medicare for all, for example, cracking down on corporate crime, a really new kind of tax system, the two parties are too hooked into the establishment, the corporate state that they can’t change. And so, if you ever put that agenda out in front of people it would be spectacular, especially if the candidate had enough money to reach those people. Let’s say a Bloomberg runs, it would be a three way race in every sense of the term, because he could write a check for $500 million.”
Bloomberg is not a left-winger who would force Democrats to the left. He's an advocate of holding down taxes on the rich and an opponent of the Occupy Wall Street movement. If Bloomberg was the Democratic nominee, Nader would be assailing him as a corporate stooge.
So why would he want to support a spoiler run for an even more moderate candidate than Obama? There is no coherent rationale here. He just has an insane belief that electing Republican presidents somehow leads to good things.