The trouble with third parties like the Greens holding the balance of power or entering coalitions is that it far too often leads them to make compromises that are perceived as a betrayal by their supporters, who then punish them in subsequent elections. Most recently, the complete collapse of the Liberal Democrats in the UK after their ill-fated coalition with the Tories. But let's not forget Joschka Fischer and Germany's Greens who are now powerless after ruling with Schröder's Social Democrats in the nineties. Alliances with mainstream parties are the kiss of death for these sorts of parties. The only viable alternative to an outright victory is to form an alliance with socialists or some other party that is ideologically closer to them.