French Socialists Left in 2002.
Despite high scores, they all universally shot themselves in the foot and ended any long term hope of ever having the kind of influence they had under Jospin's government after splitting the entire French Left. 2002 was a final nail in the PCF coffin and people like Taubira and the Greens just joined the Socialist family.
I think the Socialists themselves actually ran a decent campaign and manifesto for an outgoing government. Jospin made blunders (particularly about Chirac's age) but his views were actually far more relevant to modern politics than the Gaulliste dinosaurs opposite him. Its just he failed to tap into the main issue of insecurity that dominated the election. He was spot on though that a French President should not sell himself based on one policy, and I also think he was spot on about thinking of a market economy without a market society. Way ahead of his time.