Right now, Republicans have reason for optimism. Historically, turnout among young voters—by far the electorate’s most racially diverse generation—has plummeted in midterm elections compared with the presidential contests that have immediately preceded them. Turnout from presidential to midterm elections has also fallen more modestly among African Americans, and has slipped substantially among Hispanics as well.

Recent polling offers ominous signs for Democrats that this pattern of demobilization could persist in 2018—particularly among young people—despite the Trump administration’s relentless focus on policies that reflect the priorities of his conservative older white base, from ending protection for the Dreamers, to building a border wall, to attempting to repeal the Affordable Care Act. Though surveys show those ideas all face intense resistance among younger adults, Stanley Greenberg, the veteran Democratic pollster, told me there’s a “very real risk” that Millennial turnout could lag again in 2018.

Six months ago, that might not have worried Democrats as much, since polls showed they held an advantage across the age spectrum when voters were asked which party they preferred in the midterm elections. But in recently released surveys from both CNN and the nonpartisan Pew Research Center, voters age 35 and older split almost exactly evenly between the two parties on the generic ballot. Among voters ages 18 to 29, by contrast, Democrats held a resounding 27-point advantage in the Pew survey and an 18-point edge in the CNN poll.

Those results, in turn, reflect the persistent generation gap over Trump. In the Pew survey, only about one-fourth of adults under age 30 said they approved of Trump’s performance, far less than in any other age group. Likewise, Pew found that less than one-third of adults ages 18 to 34 said they agreed with Trump on all or many issues. By contrast, nearly half of all adults age 50 and older said they agreed with Trump on most issues, a number that rose to nearly three-fifths among older whites.

All of these contrasts underscore how much the November results may be tilted by the composition of the voters who show up. In that equation, age and race are inextricably linked: While whites represent the majority of older age groups in almost all states, non-whites are already a majority of the Millennial population in 10 states—including Arizona, Georgia, Florida, Texas, and California. In 10 more, non-whites represent at least 40 percent of the Millennial generation, according to calculations by Brookings Institution demographer William Frey.

But no more than about a quarter of eligible adults younger than age 30 have voted in any of the past five midterm elections. In 2010, voters under 30 represented just 12 percent of all voters, exit polls found, down from 18 percent in 2008. The share of ballots cast by voters under 30 likewise skidded from 19 percent in 2012 to 13 percent in 2014. Each time, the proportion of the ballots cast by seniors spiked by comparable amounts. In 2010 and 2014, the vote share cast by minorities also dropped 3 percentage points from the previous presidential races. These shifts helped trigger congressional Democrats’ landslide losses in 2010 and 2014, just two years after each of former President Barack Obama’s victories.