ADDED: Mickey Kaus considers the Bradley effect + 3 other theories:
2. Lazio Effect. No ganging up on the girl! First, Edwards turns on her in the debate. Then Obama says she's merely "likeable enough." Then the press disparages her anger, mocks her campaign and gloats over its troubles. They made her cry! And then that mean macho John Edwards goes and says the crying makes her unfit to be president...And she was also mocked — I heard it on Rush Limbaugh — for trying to get the Lazio effect going. Which would be a veritable Lazio vortex.
3. Feiler/Skurnik Effect: ... The familiar Feiler Faster Thesis holds that voters are comfortable processing information at the vastly increased speed it can come at them. Jerry Skurnik's "Two Electorate" theory holds that voters who don't follow politics are much less informed than they used to be, which causes polls to shift rapidly when they do inform themselves. Put these two together...If this theory is true — or even if the campaigns think it's true — look out! (Good for blogging, though.)
The Feiler/Skurnik Effect magnifies the significance of any events that occur in the final day or two of the campaign.
4) The Congestion Alert Effect: I remember when the Southern California transportation authorities installed a state-of-the-art series of electronic signs alongside the freeways to give motorists instantaneous warnings of traffic delays. The signs don't do that any more. Why? It turned out that when you warned drivers of congestion on Route A, they all took Route B, leading the latter to become congested instead of the former. Similarly, independent voters in N.H. were told by the press that the Democratic race was a done deal--so they voted in the closer, more exciting Republican race....A primary is not an election, so this kind of funny business is always part of the mix. I remember when my father voted for McGovern. He was damned pleased about it and assured me that's what everyone was doing. And here's the final electoral map for 1972. I was crushed. How dishonest!