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Sunday's elections in Greece have shaken markets around the world, fearful that a country suddenly thrust into political chaos won't be able to pay its crushing debts and might even exit the euro. No wonder: They also mark a leap into the unknown for Greece itself. For 35 years, two political parties have dominated the game: the conservative New Democracy (ND) party and the centrist Panhellenic Socialist Movement (PASOK). But this Sunday's national elections hit Greece like an earthquake, shifting the tectonic plates that lay beneath the surface of the Greek political landscape.

Not since the 1977 national elections has a party other than ND or PASOK emerged as one of the top two contenders. Yet, angered over a declining economy, cuts in pay and pensions, rising...