The Federal Reserve was able to help engineer a recovery from the recession, but there is little the Fed can do to change the economy’s underlying prospects. “Monetary policy can’t deal with structural problems,” Mr. Achuthan said. “The litmus test for any policy is, what impact does it have to improve productivity or demographic growth?”

America’s stagnant outlook is important: Debt is a bigger burden in slow-growing economies. Paying for a growing number of retirees becomes more onerous. How the pie is divided becomes a tougher political problem in economies that don’t grow.

This stagnation, exacerbated by the fact that most of the income gains the economy has managed to achieve have gone to the upper crust, underlies much of the anger coursing through the public this election year.

On its own, raw growth isn’t enough. But without decent growth, combined with policies to maintain low unemployment, there is little prospect of improving the fortunes of the less well-off.

While Donald J. Trump exploits that anger, his grab bag of proposals — deporting a large share of the work force; offering multitrillion-dollar tax cuts, mostly for the rich, that would only further widen inequality; blocking trade with much of the world; maybe raising the minimum wage, maybe not — would do nothing to bolster growth. But don’t worry, it will be great.

Hillary Clinton, who has put together a coherent platform focused on raising the incomes and enhancing the economic security of middle-class families, has steered clear from addressing the very real danger of low growth over the coming decades. Instead, she has promised to put her husband, who presided over the burst of growth in the late 1990s, in charge of economic policy.