Incumbents often do better than final polls

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Bill Clinton did worse than expected in 1996, right?

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The most obvious and memorable UK example is 1992. Most final polls had Labour ahead, but the Tories ended up winning the popular vote by over 7%. Although, actually, the exit poll hinted at a stronger-than-expected Tory performance - it projected them as (just) the largest party in seats.

Even in 1997, a lot of campaign polls showed Labour above 50% and the exit poll over-egged the nevertheless massive pudding.