What defines a Republican, these days? How about a Democrat? That's a difficult question to answer. After years of shifting priorities and ideologies, the beliefs of Republicans of today bear little resemblance to those of somebody of that affiliation from a decade ago, and the modern Democrat is just as far removed from recent predecessors. But one thing is clear: Republicans aren't Democrats and hate anybody who is, and Democrats feel the same about Republicans.

Identity established by mutual loathing is pretty much all there is to go on when partisans of the two factions so rapidly change positions, sometimes despising one another for holding fast to beliefs they themselves once supported. In their struggle for control of the government, it's all about loyalty and power, without any deeper meaning.

"Six in 10 Republicans say they would rather have a president who agrees with their political views but does not set a good moral example for the country, as opposed to one who sets a good moral example but does not agree with them politically. In contrast, 75% of Democrats prefer a president who sets a good moral example over one who agrees with their issue positions," Gallup reported last week. "In 1999, Republicans' and Democrats' opinions were reversed, with Republicans favoring a president who sets a good moral example and Democrats preferring one who agrees with them politically."

Of course, Republicans downplay moral issues at a time when the president from their party shows every sign of being morally crippled, just as Democrats deemphasized morals when their own occupant of the White House had his sleaziness on display. Then as now, tribal affiliation overcame any supposed principles.

The primacy of tribal affiliation has also been obvious in the course of semi-regular media pranks when partisans have been deliberately presented with misattributed quotes about public policy issues. Interviewees inevitably become befuddled when they learn that the "bad" opinion they dutifully denounced belonged, not so long ago, to the "good" side.

More broadly, the recent changes in party positions have involved "the transformation of the GOP into the party of Patrick J. Buchanan and Donald J. Trump—defined by cultural resentments, crude populism, and ethnic nationalism," as Peter Wehner puts it in The Atlantic. At the same time, "the Democratic Party is embracing a form of identity politics in which gender, race, and ethnicity become definitional" along with progressive/socialist economics that are "fiscally ruinous, invest massive and unwarranted trust in central planners, and weaken America's security."

Such rapid shifts on issues and ideology have left little time for developing a strong basis of enthusiasm for what political partisans are supposedly for, but that's left people plenty of energy left to expend on what they're against.

"We find that while partisan animus began to rise in the 1980s, it has grown dramatically over the past two decades," Shanto Iyengar and Masha Krupenkin, political scientists at Stanford, reported in a paper published last year. "As animosity toward the opposing party has intensified, it has taken on a new role as the prime motivator in partisans' political lives. … today it is outgroup animus rather than ingroup favoritism that drives political behavior."

That means politically partisan Americans are defining themselves not by what they have in common with allies, but by how much they hate their enemies. They may not have a good handle on what they're fighting about but, damnit, they're gonna fight.

Fifty-five percent of Republicans said Democrats are "more immoral" than other Americans, and 47 percent of Democrats said likewise about Republicans, as Pew Research noted last month. "The level of division and animosity—including negative sentiments among partisans toward the members of the opposing party—has only deepened" since the last survey three years ago.

Fifty-five percent of Republicans and 44 percent of Democrats say the party opposing their own is "not just worse for politics—they are downright evil," according to a YouGov survey. Thirty-four percent of Republicans and 27 percent of Democrats say the other party "lack the traits to be considered fully human—they behave like animals."

Factions that aren't really firm about what they believe, shift positions, but are dead-set in their hatred for one another to the point of dehumanization? In a weird way, such partisan animus for its own sake sounds an awful lot like the ancient rivalry between the Blues and the Greens—the chariot teams turned political parties that played such a prominent role in sixth-century Byzantine political life. As with modern Republicans and Democrats it was never entirely clear what they stood for beyond opposition to one another, but rioting between the two factions ultimately burned half of Constantinople to the ground.

When the Blues and Greens set about burning down their city, they were encouraged by senators who hoped to seize the imperial throne for themselves. They sought to take advantage of the chaos.

Nothing much has changed over the centuries.

"Partisan negativity is self-reinforcing, that is, political elites are motivated to stoke negativity to boost their chances of reelection," Iyengar and Krupenkin wrote in their 2018 paper.

Fundamentally, then, what defines Republicans and Democrats isn't programs or beliefs or ideology—it's achieving power and destroying the enemy in the process. What's done once power is achieved—beyond grinding "evil" and "immoral" enemies into dust—is secondary at best.

Since platforms and ideas don't really matter, there's no room for finding common ground or cutting deals. Opposing political factions can compromise, for good or ill, on health care bills and defense schemes. But how do you split the difference when what separates you isn't a matter of firm values or principles, but a mutual desire to seize total control and to smash all who don't wear your gang colors?

Ultimately, the only way to keep the peace is to make sure there's no prize to be won. So long as there's a powerful government over which hateful partisans fight for dominance, we're all in danger from the battling factions.