The original book, published in the Republican year of 2002, forecast accurately the groups that would make up the Democratic majority coalition that emerged in the 2006 and 2008 elections: blacks, Hispanics, gentry liberals, single women, young voters.

But as Judis writes now, that coalition has come apart. That's partly because of diminished support from Millennials and Hispanics, but mostly because of additional white working-class defections and erosion among suburbanites unhappy with higher government spending and taxes.

At the time, some commentators, including me, hailed the onset of an enduring Democratic majority. And the arguments in defense of this view did seem to be backed by persuasive evidence. Obama and the Democrats appeared to have captured the youngest generation of voters, whereas Republicans were relying disproportionately on an aging coalition. The electorate's growing ethnic diversity also seemed likely to help the Democrats going forward.

These advantages remain partially in place for Democrats today, but they are being severely undermined by two trends that have emerged in the past few elections—one surprising, the other less so.

The less surprising trend is that Democrats have continued to hemorrhage support among white working-class voters—a group that generally works in blue-collar and lower-income service jobs and that is roughly identifiable in exit polls as those whites who have not graduated from a four-year college. These voters, and particularly those well above the poverty line, began to shift toward the GOP decades ago, but in recent years that shift has become progressively more pronounced.

The more surprising trend is that Republicans are gaining dramatically among a group that had tilted toward Democrats in 2006 and 2008: Call them middle-class Americans. These are voters who generally work in what economist Stephen Rose has called "the office economy." In exit polling, they can roughly be identified as those who have college—but not postgraduate—degrees and those whose household incomes are between $50,000 and $100,000.

Some Democrats believe that the party's support among millennial voters...[however]...the Harvard Institute of Politics has found a steady deterioration in young voters' support for Democrats since its peak in 2008. "Our recent polling," the study wrote last fall, "shows that on a wide range of issues and questions, young voters ... now look very much like the electorate at large—pessimistic, untrusting, lacking confidence in government.

Middle-class voters also tend to be less populist than white working-class voters when it comes to blaming Wall Street and the wealthy for the economy's ills. As a Washington Post poll showed last October, middle-class voters are less likely than white working-class voters or professionals to agree that America's system "favors the wealthy." Many of them work for businesses where their own success is bound up with the company's bottom line. That makes them less susceptible than white working-class voters or professionals to Democratic taunts about the "1 percent."