WHERE DO THE CHILDREN GO?:

Then there are voters ages 18 to 29, among Mr. Obama's most important supporters in 2008. The roughly 23.7 million "millennials" who voted in 2008 were 18% of the electorate, up 2.9 million voters over the previous presidential race. They gave Mr. Obama 66% to Sen. John McCain's 32%, according to exit polls. This margin of roughly eight million votes was a major chunk of Mr. Obama's overall edge of 9.6 million.

But youthful enthusiasm for Mr. Obama has waned. In October 2008, 78% of voters 18-29 told Gallup they would definitely vote that year. Now it's 58%.

There's also evidence that fewer younger people are registered. A November 2011 study from Tufts University found that 43% of the decline in Nevada's voter rolls since 2008 came from voters ages 18-24. Similarly, while North Carolina's rolls rose by 93,709 over that period, more than 48,000 younger voters were dropped from the rolls, 80% of them Democrats.

Mr. Obama's lead over Mr. Romney in the latest JZ Analytics poll among voters ages 18-29 is 49% to 41%. If young voters turn out this fall in the same numbers as in 2008 and give Mr. Obama this eight-point margin, it will take 2.8 million votes from Mr. Obama's total and add more than 3.3 million to Mr. Romney's tally.

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