Representative Martha McSally, the establishment favorite for the Republican Senate nomination in Arizona, took the unusual step this week of removing her name as a co-sponsor of legislation to provide a path to citizenship for the so-called “Dreamers,” young people brought to the U.S. illegally by their parents.

Locked in a tough primary with two anti-immigration hardliners—former state Senator Kelli Ward and former Maricopa County Sheriff Joe Arpaio—McSally’s staff said in a statement that she now prefers a competing bill: one that would offer more limited protection to the Dreamers, while funding President Trump’s request for a border wall, toughening immigration enforcement, and slashing legal immigration.

McSally’s rush to embrace that conservative wish list demonstrates how Trump is steadily tugging more of the GOP toward his nativist positions on immigration. But her move, in a state where non-whites will soon represent a majority of the population under age 40, also signaled how much of the GOP strategy for surviving the midterm elections is based on a generational wager. The Republican bet is that the party can mobilize elevated turnout among their older and blue-collar white base without provoking the young and racially diverse voters who personify the emerging next America to show up on Election Day to defend it. Few things are likely to shape November’s outcome more than whether that bet pays off.

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.

Recent polls point to similar clouds rolling across the Democrats’ horizon. Pew’s survey found only one-third of adults under age 30 said they were closely following news about the midterms. Nearly twice as many adults over 50 said they had that level of engagement. Recent polling from Greenberg similarly documented very low levels among Millennials.

An electorate with fewer younger people and minorities might not threaten Democrats in 2018 quite as much as in previous midterm elections, because the party has been running unusually well among college-educated whites. White-collar whites, who have been cool to Trump, have voted in midterms at much higher levels than whites without college degrees, who make up the president’s base. Greenberg also points to signs that when Democratic campaigns have engaged Millennials, as in Ralph Northam’s victory in the Virginia governor’s race last year, their turnout has improved.

Yet Trump has unquestionably struck a powerful chord in older, blue-collar, evangelical, and rural white voters who feel threatened by the demographic and economic changes remaking America. As Republican pollster Kristen Soltis Anderson wrote last week in the Weekly Standard, “In focus groups … I often hear older voters on the right describe Millennials as a hostile force trying to take their country away from them.”

It’s highly possible that Trump will succeed in mobilizing high levels of turnout in November from those anxious voters resistant to the changing America. If the young and diverse voters who not only embrace, but also embody, those changes don’t match that intensity, they may again cede control of the country’s future to those seeking to restore its past.