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Then there was the 2004 campaign, where Mr. Zogby said Democratic nominee John F. Kerry never got a bounce and consistently polled about the same, with the election depending chiefly on how voters viewed then-incumbent Mr. Bush.

In the key states that will decide the election, Mr. Obama holds leads nearly across the board, including significant leads in Ohio and Colorado. And if the election were held today, the president would easily win re-election, according to Real Clear Politics’ projections based on polling averages.

But Mr. Obama’s lead in many of those states is thin, and Mr. Romney has put new states in play, including Wisconsin.