A critical part of the coalition that elected Barack Obama was younger voters. In 2008, voters under the age of 30, a group that was almost entirely made up of what we often call "millennials", backed Obama by a 2-to-1 margin on his way to a blowout victory.
Greenberg poll (D) only has Clinton up ONE pt (!) over Trump w white millennials, 33-32%. Wow.— Josh Kraushaar (@HotlineJosh) September 23, 2016
As people quickly noted, though, white voters under the age of 30 actually preferred Mitt Romney in 2012. In only three of the past 11 presidential races have voters under 30 preferred the Republican, according to exit polls: 1972, 1984 and 1988. In all 11, white voters have backed the Republican. In eight of those 11, white voters under the age of 30 have also backed the Republican. In 1992, it was a tie.
In fact, over that period, white voters under the age of 30 have consistently voted a lot more like white voters overall than like voters under 30 overall. Since 2000, young white voters have voted 23 points more Republican than other young voters on average, and only about 9 points more Democratic than whites on the whole.
In 2004, young white voters backed George W. Bush by 11 points, 14 points closer to how whites voted than how younger voters cast their ballots. In 2008, they were only 2 points closer to whites overall, but in 2012 that difference expanded to 17 points.
Why does Clinton continue to see an edge among younger voters in most polls? Because of her stronger support from young nonwhite voters. Over time, nonwhites have made up a larger percentage of the electorate, particularly among younger voters. Obama did well with younger white voters in 2008, but did phenomenally with young nonwhites. He won the under-30 vote in 2008 by 34 points and won it by 23 points in 2012, despite losing 17 points of support from younger white voters over that period.
One question for Clinton is if she can count on the same depth of support from those voters. It's a question far less of young white voters than young nonwhites. In most polls, the number of nonwhite voters under the age of 30 that are included are relatively small, making this hard to gauge. During the primary, polling suggested that Clinton's strength with black voters in particular was much softer among younger black voters, who were more likely to support Bernie Sanders.
This is why Clinton is eager to have Sanders out on the stump as frequently as possible. It's also why she's got Barack Obama making his pitch in Philadelphia and why she's made a concerted effort to woo younger voters.
Sure, Clinton could be doing better with younger white voters, but they typically lean more Republican than other young voters. The part of the younger Obama coalition Clinton needs to replicate is everyone else.