Why Do Polls Historically Show Dems Winning – And Then They Lose?

Actually it was posted yesterday, but his question is astoundingly on point:

Now here’s my question: When, in the last two cycles of 2002 and 2004, has any national poll of standing shown a Republican to be ahead at this point in the race where that Republican in fact lost? (You can’t cite Zogby for this proposition, as the poll must generally be recognized as having at least a shred of credibility.) While poll after poll over these years have shown Democrats ahead who in fact lost, can you recall a race where polling put a GOP candidate ahead who in fact got smacked badly on election day?

Nope. And I’m trying. Anyone with a legit exception, throw it in the comments or e-mail me.