‘One of the most brutal races in the country’ has just begun in Florida.

“In 2020, the Electoral College vote forces us to win in both places,” says Robby Mook, the 2016 campaign manager for Hillary Clinton. “But over time, because of population shifts, I think we are going to spend a lot more time in the Sun Belt.”

This long-term geographic reconfiguration reflects the shifting demographics in each party’s coalition—a transformation that Trump’s tumultuous presidency is accelerating. Trump is intensifying the GOP’s strength among older, blue-collar, and rural whites. But his style and agenda are simultaneously alienating younger and nonwhite voters, and straining the GOP’s grip on college-educated whites, especially women.

The shift toward the GOP among older whites threatens the long-term position of Democrats in a wide array of states across the country’s heartland, where those adults constitute a critical mass.

In this year’s Senate contests, Democrats appear to have stabilized their position in several of the industrial states, with incumbents generally considered strong favorites for reelection in Michigan (Debbie Stabenow), Ohio (Sherrod Brown), and Pennsylvania (Bob Casey), along with a more equivocal favorite in Wisconsin (Tammy Baldwin). But the GOP’s strength among older and blue-collar whites has still placed Republican challengers in a strong position to oust Democratic Senators Heidi Heitkamp in North Dakota, Joe Donnelly in Indiana, Claire McCaskill in Missouri, and—to a somewhat lesser extent—Joe Manchin in West Virginia and Jon Tester in Montana. All of those are preponderantly white and heavily blue-collar states where Trump remains much more popular than he is nationally.

The resilience of industrial-state incumbents such as Brown, Stabenow, and Casey makes clear that Democrats aren’t facing imminent extinction in the heartland; they are even well positioned to potentially recapture several governorships there this year. But the potential losses among the second group of senators, who are defending more rural states, underscore the likelihood that the Democratic position in the heartland will continue to erode over time, particularly as the GOP appeals more overtly to white anxiety over demographic and cultural change.

As that occurs, Democrats through the 2020s will need greater gains in the Sun Belt to fill the gap at every level—in contests for the House, the Senate, and the Electoral College. Since 2008, Democrats have already brought into their camp two Sun Belt states that reliably leaned Republican through the early-21st century: Virginia and Colorado. New Mexico, a longtime swing state, has also tilted blue, with Clinton carrying it comfortably, Democrats holding both Senate seats, and Representative Michelle Lujan Grisham, the party’s nominee, in a close contest to recapture the governorship.