Nate Silver at FiveThirtyEight.com takes a fascinating look at voting patterns by age groups. Silver took a survey done by Gallup that calculated a partisan advantage by ages. He then looked at who was president when that person turned at 18.
Silver concludes that George W. Bush may have lost Republicans an entire Generation.
It's become common knowledge that the younger generation is highly predisposed toward Democrats. (Actually, that's not quite right -- they're more predisposed against Republicans than they are toward Democrats -- but the net effects on their voting behavior are probably about the same.) What's more remarkable, though, is how sharp the increase in the partisan ID gap becomes at about age 25. People aged 26-34 are pretty Democratic, put people aged 18-25 are really Democratic.
The former group came of age in the Clinton Era. Clinton, in the public's mind, is usually regarded as an average-to-slightly-above-average President, and the voters who came of age during his Presidency are associated with an average-to-slightly-above degree of Democratic affiliation.
The 18-25 year olds, however, came of age in the George W. Bush Era. And Bush, at least the vast majority of us think, was not a good President. In fact, most of us would say, he was a really awful President. And the people who turned 18 during his tenure are associated with extremely low levels of Republican identification.
The reason this is a real worry for the Republicans is because you can still see the echo of past Presidencies on the partisan ID trends today. Popular presidents are associated with above-average levels of party support among the generation that came of age during their time in office, whereas unpopular Presidents are associated with below-average ones.