Above all, Lamb’s apparent win testified to how much energy opposition to Trump has ignited among Democrats everywhere. Lamb’s strong showing continued the pattern evident in special elections and other scheduled contests since 2016: Democratic candidates are consistently coming much closer to matching Hillary Clinton’s total number of votes in these jurisdictions than Republicans are to matching Trump’s. (By one calculation, Lamb won about 80 percent as many votes as Clinton did in the district, while Saccone generated only about 53 percent as many as Trump.) That’s consistent with the imbalances in partisan enthusiasm that usually trigger big midterm losses for a sitting president’s party—especially when his approval rating is lagging, as Trump’s is now.

Lamb’s strong showing came on terrain that has grown extremely rocky for Democrats. Whites comprise nearly 94 percent of the district’s total population—a share that’s greater than all but six districts’ nationwide. The district contains about as many college-educated whites as the national average, roughly one in three. But because the overall population is so preponderantly white, whites without a college degree still comprise roughly three-fifths of its population, many of them in small towns.

Not too long ago, so-called blue-dog Democrats held large numbers of House seats with this demographic profile. These Democrats typically combined cultural conservatism and hawkish foreign-policy views with defense of a robust government safety net—much the mix that Lamb presented. But in both the 2010 and 2014 midterm elections, a backlash against then-President Barack Obama triggered catastrophic losses among the blue dogs. Only two Democrats (Peter Welch in Vermont and Chellie Pingree in Maine) now hold seats where whites represent a larger share of the population than in the seat Lamb apparently carried. Even looking more broadly, Democrats before Tuesday held just seven of the 45 seats where whites constitute at least 90 percent of the population; Trump won 42 of those 45 districts in 2016.

Southwest Pennsylvania was an early center of the movement away from Democrats among blue-collar whites. Bill Clinton in 1992 and 1996 carried both Westmoreland and Washington, two preponderantly white counties in the district with relatively few college graduates. Al Gore in 2000 and John Kerry in 2004 each held Washington but lost Westmoreland. Obama and Hillary Clinton then lost both of them in 2008, 2012, and 2016. In fact, the GOP has won a higher share of the vote in Westmoreland than it did four years earlier in each presidential election since 1992, and it has improved in Washington all but once. Trump topped 60 percent in both.

“It’s been a slow erosion, then a collapse, like a giant ice sheet calving off a glacier,” Democratic consultant Paul Begala, who has worked in the area since the early 1990s, told me.