Joe Biden is leading in most Democratic presidential primary polls, and he has one group in particular to thank: older voters. In one recent Iowa poll, 31 percent of seniors supported him compared with a minuscule 2 percent of voters younger than 30. The Washington Post’s James Downie has labeled this generation gap as “the most important divide among Democratic voters.”

The good news for Biden? This gap works to his advantage in the primary. But there’s bad news, too: It may cost him in the general election if he becomes the nominee. To understand this, Biden can look to the experience of Hillary Clinton when she was the Democratic nominee in the last presidential election.

Why the support of seniors greatly helps Biden in the primary

Biden benefits from the support of seniors for a simple reason: They are much more likely than young people to vote in primaries. As I argue in my book, young people typically view primary elections like a party that they haven’t been invited to. In general elections, turnout among senior citizens is about 1.6 times higher than among young people. But in primaries, it’s four times higher.

If past turnout patterns are repeated in 2020, Biden can survive a poor showing among young people. Indeed, this is what happened to Hillary Clinton, who, according to the 2016 American National Election Studies, lost voters younger than 30 to Bernie Sanders by a stunning 73 to 26 percent margin, but she won the nomination anyway.

Clinton’s challenge in the general election

The risk to Biden is that he ends up in the same boat as Clinton in the November general election. One of the most important but least told stories of 2016 was the fact that young people just didn’t warm to Clinton, even after the primary was over. When the American National Election Studies asked respondents to rate the candidates on a 0-100 scale, Clinton scored 41 among respondents younger than 30. In 2012, Barack Obama had scored a 63. Indeed, the gap between views of Obama and Clinton was the largest among younger voters.

This helps explain why Clinton did worse among young voters at the ballot box, too. Exit polls found that 55 percent of young people voted for Clinton, compared with the 60 percent who had voted for Obama in 2012. The 5 percent drop in Democratic support among young voters was the most significant loss the Democrats experienced among any age group.

Clinton’s problems with young voters were well known to her campaign. Clinton’s campaign manager, Robby Mook, said that Clinton’s support among voters fell far short of the 60+ percent that they were counting on. “That’s why we lost,” Mook said.

What will young voters do in 2020?

When the Democrats rebounded in the 2018 midterm elections, young voters were also a big part of the answer. In exit polls, 67 percent of 18-to-29-year-olds voted for Democratic House candidates — 12 points more than voted for Clinton just two years before.

This bring us to 2020 and to Biden’s candidacy in particular. Even if Biden can get the nomination without much support from young people, if 2016 is any guide, he will need strong support from young voters to win the presidency.

Fortunately for Biden, right now it would appear that he is in better shape with young people than Clinton was at this point four years ago. According to the most recent YouGov national poll, Biden’s favorable-unfavorable rating among young people is 38 percent to 39 percent, whereas Clinton’s rating four years ago was 32 percent to 54 percent.

At the same time, Biden will continue to face criticism from other Democratic candidates. If younger voters consolidate behind a Democratic alternative to Biden in the primary, he may still face the challenge of getting them firmly to his side in the general election. Certainly, if Biden is the nominee, President Trump will attack him much as he did Clinton. How younger voters react to this strategy will once again be critical to the general election outcome.

Martin Wattenberg is professor of political science at the University of California at Irvine and author of “Is Voting for Young People?

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