WASHINGTON — One lesson of the midterm elections is that economic growth is losing its power to unite the country and to reduce explosive conflicts over race, religion, ethnicity, immigrant status and sexuality. This is unfamiliar. Economic progress has been a routine part of our election narratives. The presumption is that a strong economy favors the incumbent party and a weak economy does the opposite.

We believe prosperity encourages loyalty to the larger promise of America. It reflects Americans’ self-identity. Getting ahead is something everyone can achieve — at least in theory. By contrast, race, religion, ethnicity and the like speak to a splintered society.

Even a cursory review of presidential campaigns confirms the persistence of economic themes. In 1960, John Kennedy pledged to “get the country moving again.” Lyndon Johnson promised a “great society.” Ronald Reagan blamed Jimmy Carter for a sky-high “misery index” (the sum of inflation and the unemployment rate).

To be sure, there have been other issues: Vietnam, Watergate, crime and terrorism. But economic themes have always been at the forefront, or just behind. What is intriguing about the 2016 election and the recent midterms is that this relationship appears to be breaking down.

Despite many problems, the economy in 2016 seemed strong. When voters went to the polls, the unemployment rate was 4.6 percent, annual inflation was only 1.7 percent, and median household income had increased 5 percent in 2015 from 2014. Nope, not enough.

Similarly — and despite the usual midterm bias against the party of the incumbent president — the economy seemed healthy enough to help the Republicans retain control of the House. Unemployment was lower than in 2016 (3.7 percent), inflation was only a tad higher (2.3 percent). Median income has continued to advance. Nope, not enough.

Maybe it’s no longer the economy.

In a new book, “Identity Crisis: The 2016 Presidential Campaign and the Battle for the Meaning of America,” political scientists John Sides of George Washington University, Michael Tesler of the University of California, Irvine, and Lynn Vavreck of the University of California, Los Angeles, argue that the last presidential campaign was a clash of identities, to borrow a phrase from the late political scientist Samuel Huntington.

People saw their adversaries as threats to their own way of life — and, of course, were often urged to do so. Trump was proud of his ability to incite partisan crowds; Clinton was not totally blameless either, with her condescending reference to “deplorables.”

Here’s how Sides, Tesler and Vavreck characterize the 2016 campaign:

“Trump’s victory … relied on activating people’s pre-existing views of racial, ethnic, and religious minorities. … Democrats reacted against Trump’s agenda. Thus, the alignment between partisanship and attitudes about issues like race and immigration only increased, and with it the likelihood of even more divisive politics. The resulting partisan polarization is the linchpin of America’s identity crisis.”

Put slightly differently, we have a vicious circle. The anger on one side of the political spectrum feeds anger on the other. Polarization grows; people become more and more distrustful.

Political scientists Alan Abramowitz and Steven Webster of Emory University have coined the useful term “negative partisanship,” by which they seem to mean that many Americans are more fearful of what the other party might do rather than enacting their own agenda. Public-opinion polls of both parties’ nominees found that “large majorities of Democrats and Republicans truly despised the opposing nominee,” they write.

Fast forward to 2018. The Democrats seem to have done to the Republicans and Trump what the Republicans and Trump did to them in 2016. Despite the relatively robust economy, House Republicans lost badly; at this writing, Democrats have picked up 33 seats.

Americans have a tendency to think that prosperity, administered in sufficient amounts, can cure almost any social and political problem. The experience of recent elections stands as a reminder that this sort of reasoning is often wishful thinking.

Of course, the economy hasn’t permanently disappeared from political life. Given another recession (which, at some point, is inevitable) or financial crisis, its role would undoubtedly rebound.

But meanwhile, it takes a back seat to today’s hateful partisanship. The purpose of politics — or, at any rate, one purpose — is to conciliate and to cooperate. On that score, we are in a bad place.

Robert Samuelson is a columnist with The Washington Post.

Robert Samuelson