There not being nearly enough discussion of chariot racing in American politics, it is worth recalling the story of the Greens and the Blues. These were the two main racing teams of the Byzantine Empire, and they generated considerable sporting enthusiasm. A clash of their fans in the riots of the year 501 took perhaps 3,000 lives. According to the judgment of many historians, these teams did not originally have political definitions. Blues, it seems, hated Greens mainly because they persisted in being Greens, and vice versa. But over time, the teams became identified with politicians and religious movements.

When it appeared that both powerful teams were uniting to overthrow Emperor Justinian, he blocked the exits at the Hippodrome and had his troops slaughter perhaps 30,000 fans. "Within a few minutes," writes historian John Julius Norwich, "the angry shouts of the great amphitheater had given place to the cries and groans of wounded and dying men; soon these too grew quiet, until silence spread over the entire arena, its sand now sodden with the blood of the victims."

This is hardly a constructive model for dealing with excessive factionalism. But the example points to the danger of viewing politics as a team sport. Citizens can engage in civil discourse and productive compromise. Rabid fans can only be appeased by victory.

The most common form of criticism I receive from conservatives when I point out the tension between Trumpism and actual conservatism is that I am helping the other side in a win-or-lose contest. This approach to politics is cultivated by President Trump, who emphasizes solidarity with his political and cultural team to draw attention away from his own incompetence, ignorance and ideological heterodoxy. Much of his public rhetoric is in the form of trash talk, which many Americans seem to prefer to the boring language of policy disagreement or to the highfalutin language of shared aspiration.

Trump not only enjoys negative polarization, he depends on it. If your political reference points are decency, character and a commitment to the common good, the president is an ongoing disaster. If you believe that politics is the zero-sum struggle between cultural teams or tribes, then you want the most vicious bully on your side. There is no serious argument for Trump based on his own virtues. He only makes sense if politics is a cruel and bitter game.

The implications? At one level, a politics based on team loyalty ceases to serve political purposes. It may be entertaining -- to those who find democratic decline a hoot -- but it makes the building of working coalitions to confront specific problems more difficult. Anyone who wishes to cooperate with elements on the other side on, say, education reform, or health reform, or entitlement reform is viewed as giving aid and comfort to the enemy. If the main standard in politics is the victory or loss of the tribe, then the task of passing laws to make conditions better becomes secondary and suspect.

The prevalence of team groupthink has undermined the political functions of a variety of institutions. Think of the judicial confirmation process, which has been a spectacle of raw partisanship for decades. Senatorial deliberation on qualifications and temperament has devolved into a conflict waged by competing political campaigns.

Yet there is also a deeper level of harm to national ideals. Political scientist Amy Chua describes America as a "super-group." People of different ethnicities, races, religions and cultural backgrounds can all have full membership. No one is forced to abandon his or her identity at the door. But the American super-group is held together, argues Chua, by "a strong, overarching collective identity." And this model is relatively rare in history. It is unusual, says Chua, "to have both an extremely diverse, multiethnic population and a strong overarching national identify capable of binding the people together."

This source of strength also makes America especially vulnerable to identity politics of left or right. When the overarching identity is weakened by declaring human differences to be primary, or when some subgroup of lighter-skinned people claims exclusive ownership of our national ideals, something rare and valuable moves toward history's disturbing norm of tribalism. It is playing with a form of fire that has burned time after time, through nation after nation.

These are the stakes when citizens become fans and turn the honorable calling of democratic politics into a destructive game.

Michael Gerson's email address is michaelgerson@washpost.com.