When Hillary Clinton delivers a speech aimed at Millennials in Philadelphia on Monday, she will be confronting perhaps the most persistent weakness in her career as a national candidate.

Clinton struggled among Millennial voters in her 2008 primary campaign, her 2016 primary campaign, and in the 2016 general election. Against Donald Trump, Clinton has two big advantages—a policy agenda that polls show largely matches Millennials’ own preferences, and an opponent even more unpopular with them than with the public overall. But she also must overcome her own long history of failing to connect with this growing group of voters—a failure that is increasingly worrying Democrats as the overall race tightens.

“This could very easily be the difference between winning the election or not,” said Andrew Baumann, a Democratic pollster who is regularly polling Millennials during this campaign. “If she ends up with them at 50 percent [of the vote] or 55 percent or 60 percent, those are hugely different scenarios.”

In simplest terms, Clinton’s problem is that large numbers of Millennials have never warmed to her as a national candidate.

In 2008, the cumulative analysis of all exit polls conducted during the Democratic primary found that Millennial voters preferred Barack Obama over Clinton by a solid margin of 58 percent to 38 percent. Eight years later, running against Bernie Sanders, a septuagenarian socialist who was virtually unknown as a national figure when the race began, Clinton lost Millennials even more resoundingly, 71 percent to 28 percent, according to a cumulative analysis of exit poll results by the ABC pollster Gary Langer. Sanders beat Clinton among Millennials in all of the 27 Democratic primary and caucus states with exit polls this year, except Mississippi and Alabama. Even in states she won comfortably, like New York and Pennsylvania, Sanders beat her by two-to-one or more among the youngest voters.