Voter data by TargetSmart Communications.

In late February, Barack Obama stood before a room of his party’s governors at a Washington fund-raiser and offered a new explanation for the Republican rout that claimed the jobs of more than 750 Democratic officeholders around the country in 2010. At the time, the president had described the outcome, simply and indelibly, as a “shellacking,” but here he ventured a deeper analysis. “We know how to win national elections,” he told the crowd. “But all too often it’s during these midterms where we end up getting ourselves into trouble, because I guess we don’t think it’s sexy enough.”

Beyond the narcissism implied—the suggestion that any ballot without his name on it lacks a certain magnetism—Obama was onto something. Current conventional wisdom holds that Democrats’ prospects this November are grim. After the obligatory acknowledgment that the party in the White House almost always loses ground in off-year elections, the most commonly cited reasons are situational—the botched Obamacare rollout, a zealous conservative base, the fact that these midterms follow a redistricting process largely controlled by Republicans, the preponderance of competitive Senate races in states that lean red. And yet Obama’s diagnosis of Democrats’ midterm woes comes closer to the truth. The party is suffering from a chronic condition, not a short-term malaise.In fact, the very phenomenon that sustained Obama’s own victories is the one that may doom his party in midterm elections for the rest of his life. The dynamic so cripples Democrats’ off-year performance that in 2018, a President Jeb Bush or Rand Paul could see Republicans actually pick up seats.

A decade ago, Obama memorably rebutted the trope that the United States could be neatly cleaved into a red and a blue America that pits coastal liberals against inland traditionalists. But in one very measurable and consequential sense, there are two Americas. There is the America that votes in presidential elections, which has helped Democrats win the popular vote in five out of the last six cycles and supports the view that Hillary Clinton can continue that streak should she run. Then there is the America that votes more regularly, casting ballots in both presidential and midterm years, which led to the Republican wave in 2010 and gives its party’s leaders reason to be so sanguine about their odds this time around.

There are about 127 million people in that first category, and among their number is the ascendant coalition—young and diverse, urban and mobile—that now gives Democrats a huge advantage in presidential races. But only 78 million of those people, or about 40 percent of the country’s voting-age population, belong to the group that goes to the polls every two years, and those regular voters carry a considerably more conservative cast. (The number of unregistered voters is almost as large.)