The challenge is especially urgent for Democrats because Trump divides younger and older Americans so sharply. Though Trump showed strength among blue-collar white Millennials, he carried just 36 percent of young people overall last November. Polls show he’s lost ground since. Both the CNN/ORC and NBC/Wall Street Journal surveys released last week found his approval rating among adults ages 18 to 34—almost exactly the Millennial generation’s boundaries—falling below 30 percent. That’s much lower than his ratings among older adults, especially those 50 or older.

Polls have also found that over three-fourths of Millennials oppose both Trump’s Mexico border wall and his push to repeal Obama’s climate-change agenda. Eliminating funding for Planned Parenthood, cutting taxes for top earners, barring Syrian refugees—each Trump priorities—all face preponderant Millennial opposition in surveys.

That hostility has produced a pronounced Democratic lean in early tests of 2018 attitudes. Millennials said they preferred Democrats for Congress by crushing margins of nearly 30 percentage points in both the NBC/Wall Street Journal and CNN/ORC surveys. That’s more than double the party’s advantage among younger voters in NBC/Wall Street Journal polls from 2010 and 2014. By contrast, Americans 35 years old or more divide exactly evenly between the parties on that ballot test in the CNN/ORC survey and slightly prefer Republicans in the other.

With Millennials poised to eclipse baby boomers as the largest generation of eligible voters in 2020, their widespread antipathy toward Trump’s insular vision for the country looms as a long-term GOP challenge. Democrats, though, face the bigger near-term problem: motivating them to vote in 2018. As Bonier suggests, the balance between how many younger and older voters turn out could decide control of the House.

Figures compiled by CIRCLE, an institute that studies voter engagement at Tufts University, found that turnout among eligible voters ages 18 to 29 plummeted by more than half from the 2008 presidential election to the 2010 midterms and by that much again from 2012 to 2014. That decline was proportionally far greater than among older generations. (It was also greater than the falloff among minority voters from presidential to midterm elections.)

The result is that while young people ages 18 to 29 comprised nearly one-fifth of all voters in 2008 and 2012, their share collapsed to about one in eight in 2010 and 2014. Seniors, in turn, cast considerably more of the total vote in those midterms than in the presidential years. If that dynamic persists, it will measurably boost Trump and the GOP. In the CNN survey, over seven in 10 Millennials said they want candidates who will oppose Trump. A slight majority of seniors said they want candidates who will support him.