That resistance shadowed every item on the Democratic agenda during former President Barack Obama’s first two years, from health care to climate change. “Nothing was easy,” says Henry Waxman, the veteran former legislator who served as chairman of the powerful House Energy and Commerce Committee over that period. “I remember complaining to Pelosi that she was putting too many blue dogs on the committee and she said, ‘You’ll have to do what we all have to do: compromise.’”

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Compromise was necessary back then because the party still relied on blue dogs to keep its control of the House, though their districts were increasingly attracted to Republicans in presidential elections. CityLab has developed an innovative system for ranking House districts on an urban-rural scale based on the density of their population and other factors. Its analysis found that, in 2009, fully 89 of all House Democrats, or 35 percent, held seats in the two most rural categories. My own previous analysis of the 2009 class found that 76 Democrats represented heavily blue-collar seats that had fewer minorities and fewer white college graduates than the national average.

The moderate-to-conservative blue dogs centered in those rural and blue-collar seats were a consistent source of unease about the aggressive agenda Democrats pursued with unified control of the White House, Senate, and House under Obama. In 2009, 44 Democrats voted against the cap-and-trade climate-change bill that Waxman and Pelosi steered through the House; the next year, 34 voted against final passage of the Affordable Care Act. (“I thought that [cap-and-trade] was difficult but health care would be easy,” recalls Waxman, who shepherded both bills through his committee. “But even health care wasn’t easy.”) After those two votes, the blue dogs’ reluctance to take more risky votes helped convince Pelosi and the White House to abandon consideration of comprehensive immigration reform, much less any new gun-control measures.

That caution couldn’t stem the tide. The rural and blue-dog Democrats were living on borrowed time as small-town, evangelical Christian, and blue-collar whites were becoming more reliably Republican. In the 2010 midterm election, the GOP hunted the blue dogs nearly to extinction: Fifty-one of the 89 rural Democrats CityLab identified were defeated as the GOP surged into the majority.

Critically, the Democrats rebuilt their majority in 2018 without relying on such inherently unstable turf. Instead, the new class has the party advancing into different terrain. Only 35 Democrats in the new caucus hold seats in CityLab’s two most rural categories, according to figures shared by David Montgomery, who developed the ranking system. That represents only about one-fifth of all of those seats—and just 15 percent of the total Democratic caucus. By contrast, 200 of the new Democrats (or 85 percent) hold seats in CityLab’s four most urban and suburban categories. Those districts include almost all of the Republican-held seats that Democrats captured last fall. In the three most urban categories of House seats, Democrats now crush Republicans, 149 to 16.