Democrats and Republicans dislike each other more than ever before.

Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE won the White House, in part, by creating an identity for his followers — distinguishing sharply between an aggrieved “us” and a guilty “them.”

Disregarding the definitional problems I’ve treated before, America feels deeply polarized.

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How did that come about?

Numerous hypotheses have been advanced, from a decline in D.C. schmoozing by politicians to the rise of ideologically based news.

I want to offer another, purely speculative, thought.

What if “us” needs them? Human beings seem to have a natural proclivity to create in-groups, which implies the need for out-groups.

Dividing people into groups comes naturally to our social species. Psychologists have been able to produce major features of group effects — favoritism and loyalty to ingroups, along with hostility to outgroups — with great ease, using trivial bases of group identification.

Informing people that they were divided based on whether they seemingly under- or over-estimated the number of dots on a slide, or their favorite modern artist, or even randomly, by the public flip of a coin, all induced significant group effects, including trust, affection, cooperation and more generous rewards for in-group members, as well as negative responses to out-groups.

Group conflict hardens the boundaries between groups and reinforces the identities.

In the 1940s, perhaps the sharpest line of demarcation was between the Allies and the Axis, with whom we were at war (Nazi Germany, Japan, etc.).

World War II divisions were quickly replaced by Cold War conflicts.

We felt relatively united as Americans because we faced a foreign foe, enemies that truly threatened us. Americans were “us” and our enemies were “them.”

With the fall of the Berlin Wall and the dissolution of the Soviet Union, our existence was no longer in peril.

“They” no longer loomed large in our thinking. But “us” still needed a “them.”

Absent a threat from overseas, we needed to find one here at home. Adherents of the opposing political party, immigrants and Muslims filled the bill, while the left found its own enemies.

As I said, rank speculation. But there is a scintilla of evidence for the proposition in a study by the University of Pennsylvania’s Matthew Levendusky, which found that increasing the salience of American identity, decreased partisan animosity.

Emphasizing the broader identity that unites us reduced the relative importance of other identities that divide us.

Having participants in an experiment read an article about the strengths of America and then write a paragraph explaining what they like best about the country was sufficient to prime their identity as Americans.

That in turn lessened negative views of the opposing party.

Levendusky also found that respondents interviewed around the Fourth of July, when American identity runs strong, felt better about the presidential candidate of the opposing party than those questioned before and after the Fourth.

Of course, the same dynamic played out after the Sept. 11 attacks.

On Sept. 10, Gallup put President George W. Bush’s approval rating at 51 percent. Four days later it was 86 percent. Democrats went from 27 percent approval to 78 percent.

Honestly, that wasn’t because of the wonderful things the president did during those harrowing days.

Rather, the attacks heightened our identities as Americans. Flags flew everywhere. Patriotism was in fashion, and our other identities, say as Democrats and Republicans, suddenly became far less salient.

We all have multiple identities. Highlighting some can reduce the immediate significance of others.

Facing a threat, our American identity takes over and internal conflicts among Americans are muted.

When the threat recedes, our internal conflicts come to the fore.

It would be a shame if the only way we can come together as Americans is in response to menace or tragedy.

Mellman is President of The Mellman Group and has helped elect 30 U.S. Senators, 12 Governors and dozens of House Members. Mellman served as pollster to Senate Democratic Leaders for over 20 years and as President of the American Association of Political Consultants.