In 2006, Nancy Pelosi muzzled her liberal inclinations to recruit and elect her “Majority Makers”—a crop of moderate and conservative Democrats who won Republican districts and delivered control of the House for the first time in 14 years.

Democrats in 2006 also claimed the Senate, with savvy victories in states like Montana and Virginia. The party thumped Republicans in gubernatorial races, winning in the South (Arkansas), the Mountain West (Colorado), and in Ohio (for the first time since 1991). A vibrant candidate Obama further boosted Democratic ranks in 2008.

By 2009, President Obama presided over what could fairly be called a big-tent coalition. The Blue Dog caucus had swelled to 51 members, representing plenty of conservative America. Democrats held the majority of governorships. Mr. Obama had won historic victories in Virginia and North Carolina. The prediction of liberal demographers John Judis and Ruy Teixeira’s 2004 book, “The Emerging Democratic Majority”—lasting progressive dominance via a coalition of minorities, women, suburbanites and professionals—attracted greater attention among political analysts.

It took Mr. Obama two years to destroy this potential, with an agenda that forced his party to field vote after debilitating vote—stimulus, ObamaCare, spending, climate change. The public backlash, combined with the president’s mismanagement of the economy, has reversed Democrats’ electoral gains and left a party smaller than at any time since the mid-1990s.

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