Gallup has some fascinating data out, based on more than 120,000 interviews they've completed over the past four months, on the way that partisan identification breaks down by age:
Democrats, somewhat unsurprisingly, have the largest partisan ID advantage among Gen Y'ers, followed by among Baby Boomers. Republicans do relatively well (although are still at a net disadvantage) among Generation X'ers.
What's interesting, though, is what happens when we look at not these abstract generational categories, but rather at the following question: who was President when you turned 18? As annotated in the chart below, the popularity -- or lack thereof -- of the President when the voter turned 18 would seem to have a lot of explanatory power for how their politics turned out later on:
Partisan ID Gap, Based on Identity of President When Voter Turned 18
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. Moving backward in time:
George H.W. Bush, a roughly average President who was generally quite popular until roughly the last 12 months of his tenure, is associated with a slightly above-average amount of Republican support.
Reagan, a highly successful President who was popular throughout most of his term and may be even more popular today, is associated with a considerably above-average amount of Republican support.
Carter, a mediocre-to-average President, is associated with slightly below-average levels of Democratic support.
Ford, a mediocre-to-average President, is associated with average or slightly below-average levels of Republican support.
Nixon, who was reasonably popular before the Watergate Scandal broke but who is generally regarded as a very poor President today, is associated with below-average levels of Republican support.
Johnson, whose complicated time in office is generally regarded today as having been an above-average Presidency, is associated with generally above-average levels of Democratic support.
Kennedy, who was very popular throughout his brief tenure in the White House, is associated with above-average levels of Democratic support. (You can almost see the spike in popularity among 64- and 65- year olds, who would have been about 17 when Kennedy took office.)
Eisenhower, a popular and effective Republican president, is associated with significantly above-average levels of Republican support.
Finally, we get to Truman and Roosevelt, where things seem to break down a bit. Truman is regarded quite favorably by historians today but was unpopular for much of his tenure; he is associated with average-to-slightly-below levels of Democratic support. The numbers then bounce around a bit for FDR, perhaps because there aren't all that many people in their mid-80s and so the sample sizes are small.
In general, however, this points toward the idea that partisan identification -- while not exactly being "hard-wired" -- can be quite persistent as the voter moves through her lifecourse. Voters who came of age during the eight years of the Bush Presidency are roughly eight points more Democratic than the rest of the country; that advantage could be worth an extra point or two to Democrats throughout the next half-century.