The trend was underlined last week by the release of a flurry of NBC/Marist College/Wall Street Journal polls that showed Trump even with Clinton in Ohio and just narrowly behind in Iowa, while lagging by virtually identical margins of six to nine percentage points in Virginia, North Carolina, and Florida in the Southeast, and Colorado in the Southwest.

Political strategists in both parties generally rank those four states, plus Nevada, as the five true swing states in the Sunbelt. New Mexico, a sixth Sunbelt state, was competitive during the George W. Bush years, but both sides now place it safely in the Democratic camp. Together, the five most competitive Sunbelt states offer 72 Electoral College votes (led by Florida with 29).

Likewise, strategists identify five Rustbelt states as key battlegrounds: Ohio, Iowa, Wisconsin, Pennsylvania, and Michigan. (The final state that both sides have treated as competitive in recent elections is New Hampshire, which is geographically distant from both clusters, but demographically and economically more closely resembles the battlegrounds in the Rustbelt than the Sunbelt.) The five Rustbelt battlegrounds offer 70 Electoral College votes (led by Pennsylvania with 20).

Just gaining ground in the Rustbelt won’t be enough for Trump if he can’t contain his losses in the Sunbelt. Even if Trump flipped all five of the Rustbelt swing states—each of which backed President Obama over Mitt Romney in 2012—he would still lose the election if he can’t win any of the five Sunbelt swing states, assuming no other states changed hands.

The shifting balance in these two key groups of swing states reflects, above all, the changing demography of each party’s electoral coalition. The working-class white voters underpinning Trump’s support represent a much larger share of the vote across the Rustbelt than the Sunbelt, whether using data from the exit polls or the Census post-election survey (which consistently identifies non-college whites as a large share of the electorate than the exit polls do). The Census results identify non-college whites as at least 54 percent of 2012 voters in all five Rustbelt states—and no more than 44 percent in any of the Sunbelt states. And except for Florida, the Rustbelt states, as a group, are also generally older, at a time when Republicans are dominating among whites older than 45.

Contrasting dynamics are molding the Sunbelt states. Diversity is the most important. The non-partisan States of Change project has projected that from 2008 to 2012 the minority share of eligible voters will rise by more in each of the Sunbelt swing states than in any of the Rustbelt battlegrounds, with the biggest increases registered in Nevada (a stunning 7.3 percentage points), Florida (4.5 points), and Colorado (4.3 points). In each of those states except Colorado, the States of Change model projects that the white share of eligible voters will dip below 70 percent in 2016. By contrast, the model projects that whites will still comprise at least 80 percent of eligible voters this year in all of the Rustbelt swing states except Michigan (where it will dip only to 78 percent).