The party conventions were rightly characterized as either relentlessly downbeat (Republican) or fervently optimistic (Democratic). The Republican view rests in part on poll numbers that show a solid majority of Americans think that the country is headed in the wrong direction. What has caused this malaise?

Perhaps it’s the country’s dismal economic growth rate: From 1947 to 2007, the economy grew at 3.4 percent per year. But over the last four years, gross domestic product expanded at only a sluggish 2.0 percent. In 2016, G.D.P. has barely reached 1 percent growth.

In the 2012 election, voters credited President Obama with the recovery from a calamitous recession. But they are now right to ask, “Is this all there is?” In 2016, voters expect Hillary Clinton and Donald J. Trump to provide solutions to the economy’s languid growth.

Yet the widespread unease goes beyond slow growth and the accompanying wage stagnation. Underlying deep-seated voter anxiety follows trends that are decades in the making, including fears of insecurity that plague millions of Americans.

Some of these problems can be tackled with bold presidential policies. But there are limits, and other problems may lie beyond the realm of feasible solutions. ...