Sen. Bernie Sanders (I-Vt.) during a rally on July 20 in Kansas City, Kan. (Photo by Dan Videtich For The Washington Post)
Columnist

Students of civil wars know they follow a well-established dynamic. A sect, radicalized by militant leaders, commits atrocities against another sect. The targeted group retaliates with its own massacres. And thus begins a cycle of violence. Think of Serbs vs. Bosniaks in the former Yugoslavia, Sunnis vs. Shiites in Iraq, Hutus vs. Tutsis in Rwanda. I fear that a similar process of mutual radicalization is occurring in America – minus, mercifully, widespread physical violence. Political attacks — such as the death of Heather Heyer in Charlottesville last year, or the fight that broke out in New York City last week between progressive protesters and a right-wing hate group called the Proud Boys — are still isolated occurrences. For now.

President Trump is the prime culprit. He claims Democrats are traitors who want to open the borders to murderers and rapists, who perpetrated an “evil” hoax to stop the Supreme Court nomination of Brett M. Kavanaugh and who are determined to turn America into another Venezuela – a bankrupt socialist dictatorship. While accusing the opposition of mob rule, he has incited his followers to scream “Lock her up” – first against Hillary Clinton, and now against Sen. Dianne Feinstein (D-Calif.), neither of whom has been charged with any crime. Some of Trump’s mini-me’s are even worse. Scott Wagner, the GOP candidate for governor in Pennsylvania, threatened his Democratic opponent, Gov. Tom Wolf: “You better put a catcher’s mask on your face because I’m going to stomp all over your face with golf spikes.”

The extremism of the right has provoked a predictable backlash from the left. Kavanaugh’s confirmation hearing was interrupted repeatedly by angry protesters, videos are circulating on right-wing websites of progressive activists banging on the Supreme Court doors, and Republicans such as White House aide Stephen Miller and Sen. Ted Cruz (R-Tex.) have been hounded at restaurants. To stop Kavanaugh, some Democrats weren’t satisfied with citing Christine Blasey Ford’s highly credible testimony — they also embraced unsubstantiated charges from a woman, represented by headline-chasing attorney Michael Avenatti, who claimed that Kavanaugh had been complicit in gang rapes in high school.

I got a taste of leftist anger when I went on NPR’s “On Point” last Thursday as part of my book tour to discuss my disenchantment with the right. One caller insisted that Trump wasn’t another Hitler — he was worse! Another listener emailed that I could never be forgiven for having been a conservative: “I truly hope you die in agony as a result of the climate change you promoted.” Two days later, the Intercept, the Breitbart of the left, wrote that I should be permanently exiled from the “sanctuary of politics” because of my (now recanted) support for the Iraq War — a position I shared with then-Sens. Joe Biden, Hillary Clinton and Charles E. Schumer, among other Democrats.

This is the same kind of blind fury that drives so many Trump followers — and some leftist leaders are as willing as Trump to pour fuel on the partisan fire. When the administration began locking up the children of undocumented immigrants — truly an outrage — Rep. Maxine Waters (D-Calif.) said protesters should harass Cabinet members whenever they go out in public. After the Kavanaugh vote, the liberal activist Kat Calvin tweeted that Sen. Susan Collins (R-Maine) should never “have a moment of peace in public again.”

More recently, Clinton told CNN: “You cannot be civil with a political party that wants to destroy what you stand for, what you care about.” And former attorney general Eric Holder rejected Michelle Obama’s slogan, “When they go low, we go high.” “When they go low,” Holder said, “we kick ’em. That’s what this new Democratic Party is about.” (He later clarified he wasn’t advocating violence.)

Here’s what else the new Democratic Party is about: promoting impractical proposals with little analysis or detail. Sen. Bernie Sanders (I-Vt.) advocates breaking up big banks but can’t answer basic questions about his plan. Nor has he offered any realistic way to pay for his grandiose ideas, which include Medicare-for-all, free tuition at public universities and a federal jobs guarantee. Yet these plans — the leftist version of “build the wall and make Mexico pay for it” –have been endorsed by leading Democratic presidential aspirants. Another idea popular with the grass roots is to abolish Immigration and Customs Enforcement, even though this would simply mean that some other agency would enforce immigration laws. As Holder said, “I don’t think that, substantively or politically, that makes a great deal of sense.”

Democrats will not defeat Trump by becoming the Trumps of the left. The largest group of voters in America is independents (we make up 42 percent of the electorate), and few of us are likely to be attracted by intemperate outbursts and outlandish proposals. Progressives are no mob, but if they abandon civility, they will be playing into Trump’s hands by allowing him to portray himself as the last line of defense against the Hugo Chávezes of North America.

Even if such tactics can mobilize a progressive constituency and deliver short-term political victories, think of the long-term cost. We are already so tribalized that we see those with opposing viewpoints not as fellow citizens with whom we must compromise but as enemies who must be destroyed. There is a real danger that the social fabric could rip altogether, leading to violence in the streets, unless one side is principled enough to practice heroic self-restraint in the face of grievous provocation. And we know it won’t be the Trumpified GOP.