MANY A PUNDIT accused the 2014 midterm election of being a “Seinfeld election” — that is, an election “about nothing.” Certainly it lacked one clearly dominant issue, in the way that, say, the 2006 midterm was a referendum on President George W. Bush’s policy in Iraq. But if the just-completed campaign had no theme, it did have a context: People went to the polls — or, in massive numbers, stayed home — amid the increasingly widespread realization that, despite the U.S. economy’s gradual recovery from the “Great Recession,” middle-class family incomes are still not growing very much.

In fact, average income for the bottom 90 percent of households has barely grown at all, in real terms, over the last four decades, according to Labor Department data. In 1973, the bottom 90 percent got 68 percent of national income; in 2013, 52 percent. The economy grew, thanks to higher productivity, but the top decile captured a disproportionate share.

If you believe that democracy’s social foundation is a strong middle class, this trend is worrisome not only for its human cost, but also for the threat it poses to U.S. political health. William Galston of the Brookings Institution has described this breakdown of the postwar “liberal democratic bargain” — support for moderate politicians in return for steadily growing, widely shared prosperity — and identified it as “the one issue that more than any other will define our country’s prospects.”

And so our first point is that the just-completed election would have been much more about “something” if both parties had addressed this trend far more directly. Our second point, though, is that honest political dialogue, as opposed to populistic rhetoric, begins by acknowledging how difficult it will be to reverse middle-class income stagnation. It is, after all, a trend which afflicts all industrialized democracies. In fact, the United States is doing better now than either Europe or Japan; our healing labor market is producing jobs at a clip that should soon lead to more rapid wage growth.

Yet the recovery affects only the cyclical aspect of middle-class income stagnation, not the far more powerful structural aspect. President Obama’s top economic adviser, Jason Furman, noted in a Nov. 5 speech that three factors determine middle-class income: economic growth, income distribution and labor-force participation, all of which operated less favorably for middle-income Americans after 1973 than before it. Mr. Furman’s most alarming data point was the decline in labor force participation by prime-age workers — those between 25 and 54 years of age — especially men. Only 88 percent of prime-age men are in the labor force, down from 95 percent in 1973.

Perhaps the golden age of robust, widely-shared middle-class earnings between 1948 and 1973 was somewhat anomalous, bound to end once competitors in Europe, Japan and, later, China began to challenge U.S. economic primacy. Nothing in the last four decades refutes the basic case for flexible, innovative, American-style capitalism or points to a better alternative; to the contrary. Even so, government and the private sector must find new ways to manage that model — to stimulate growth, equitable distribution and work effort — because if the system doesn’t work for the middle class, it really isn’t working at all.