I think if the Dems nominated Kucinich, there is an outside chance that the Greens would throw their support to the Dems next year, just to be rid of Bush. But I doubt that.

I don't see Kucinich party switching, as he is pretty grounded in the Congressional Political Caucus, and was already elected on a party ticket. Some men are so ambitious to run for the presidency, like John Anderson and Pat Buchanan (who had been Republicans, as I recall), or Strom Thurmond (a Democrat at the time of his leap) that they make a leap to a third party ticket.