The Harvard Institute of Politics' latest poll of the Millennial generation, Americans between ages 18 and 29, provides further information on an interesting and potentially important Generation Gap. Not the Generation Gap between Millennials and their elders -- Millennials voted 60 percent to 37 percent for President Obama in 2012, while their elders (extrapolating from the exit poll) voted 49.3 percent to 48.8 percent for Mitt Romney -- but between younger Millennials (18-24) and older Millennials (25-29). In the 2012 election, there was no significant difference between them: Older Millennials voted 60 percent to 38 percent for Obama, younger Millennials 60 percent to 36 percent.

But the Harvard IOP poll shows Obama approval among older Millennials at 48 percent and among younger Millennials at 45 percent. That's in line with differences between the two age groups over the years. There is an even bigger, and statistically significant, difference between the two age cohorts on Hillary Clinton's favorability: 57 percent among older Millennials, 47 percent among younger Millennials. In addition, there is probably a similar difference among likelihood of voting in 2014. The Harvard IOP people don't provide the data here, but they do report that stated likelihood of voting is greater among Republicans, conservatives and whites than among Democrats, liberals and blacks and Hispanics.

Another interesting -- and to my Washington Examiner colleague Philip Klein, counterintuitive -- difference between the Millennial age cohorts is on marijuana legalization. Older Millennials support legalization by a 50-percent to 28-percent margin. Younger Millennials are evenly split, 38 percent for, 39 percent against. Interestingly, whites support legal marijuana 49 percent to 32 percent, while blacks and Hispanics are basically evenly split.

How to explain these results? There are higher black and Hispanic percentages among Millennials than among older Americans, and there is likely a higher Hispanic percentage (and perhaps a slightly higher black percentage) among younger Millennials than older Millennials. That should tilt younger Millennials toward Democrats -- but it doesn't. My working hypothesis is that attitudes continue to be affected by one's first years of interest in politics. Older Millennials were born between April 1984 and April 1989; most reached age 20 during George W. Bush's second term. Younger Millennials were born between April 1989 and April 1996; they reached age 20 while Obama was president. So perhaps the attitudes of older Millennials were shaped by the perceived unsuccess of Bush and those of younger Millennials by the perceived unsuccess of Obama. Just a theory, and the differences between the two generations are not great; but it seems to fit the data. And it undercuts, at least a bit, the supposition so widely shared in the aftermath of the 2008 election that the heavy Democratic preference of the Millennials would place the Republican Party in minority status for many years to come.