Probably the Left's biggest weakness is it's lack of willingness to market it's ideas to its audience. "Dictatorship of the Proletariat" comes to mind.
This is a particularly telling demonstration of the power of marketing. A complete dominance shift depending on how the 1967 border question is posed. People are predisposed to being on the side of whatever sounds like peace in the region, "Israel being able to keep the hills around its own airport" sounds like peace to the average person.
Not even Hasbara can educate everyone, it's actually getting worse at it. Why does the Left think it can educate everyone?