The youth vote is a pillar of the modern Democratic Party coalition. It was essential for former President Barack Obama’s 2008 and 2012 victories, and to Hillary Clinton earning a plurality of the popular vote. While voters under the age of 35 seem to be a lock for Democrats, they are less than enthusiastic about supporting Joe Biden, and it could become a serious problem in the fall.

During the Democratic primary, young voters across the country flocked to Sen. Bernie Sanders, even in states that overwhelmingly voted for Biden. In Alabama and South Carolina, where Biden won 63% and 49% of the vote, exit polls showed he still lost voters under the age of 30 by a double-digit margin.

Clinton experienced a similar pattern during the 2016 Democratic primary, yet by the time she clinched the nomination, polls showed that young voters preferred her far better than Donald Trump. Polls conducted by Fox News, Bloomberg News, the Economist/YouGov, ABC/Washington Post, and Monmouth University right before the general election showed Clinton with a 21-point lead among voters under 35. She ultimately won that age group by 19 points, 55 to 36, with 9% of young voters choosing a third-party candidate.

So far, Biden hasn’t had the same luck as Clinton with wooing young voters.

His first problem is they didn’t show up en masse in 2020, not even for Sanders. A study by The Harvard Institute of Politics found that youth turnout was either down or flat in eight of the first 12 primary contests, including in crucial swing states such as New Hampshire and North Carolina. This happened even though Sanders’s campaign designed its message and platform to cater to young progressives across the country.

Democratic insiders have been sounding the alarm about Biden’s trouble with young voters since March, telling NBC News that it reminded them of Hillary Clinton. That problem hasn’t gone away, even as he’s become the presumptive nominee. Recent polls conducted by Fox News, Monmouth, CNN, Quinnipiac, Selzer, and ABC/Washington Post found that Biden leads President Trump with voters under 35 by an average of just 12 points, 49% to 37%.

Further complicating the situation for Biden is that a large portion of Sanders’s voters still show no signs of warming up to Biden. A Morning Consult poll taken on April 7 found that a plurality of Sanders supporters under the age of 35 held an unfavorable view of Biden. A recent ABC/Washington Post poll found that 15% of Sanders supporters plan on voting for Trump in the general election. Many more are saying that they are undecided or might not end up voting at all. That poll was conducted before Trump announced he was waiving student loans for at least six months but possibly through the end of the year, a significant issue for young voters. There are even rumors that Trump may go forward with some form of student loan forgiveness.

The Biden campaign has spun into a defensive mode, with its candidate locked in his house making campaign videos, his age entirely on display. Trump’s numbers are improving substantially from where they were four years ago with younger millennials and Generation Z. If Biden plans on winning the White House, he will have to pivot his campaign toward young voters or find a different coalition than the one that pushed Obama to victory 12 years ago.

Ryan Girdusky (@RyanGirdusky) is the author of the book They're Not Listening, How the Elites Created the National Populist Revolution.