We’ve watched the Portland movie before. It doesn’t end happily. The idealistic protesters in the streets overplay their hand, tolerate violent provocateurs in their midst and eventually get crushed. The good guys lose; the bad guys win.

Over the past decade, I’ve seen this depressing scenario play out around the world, as pro-democracy activists marched courageously but unwisely into confrontations that eventually proved ruinous. Nonviolent protest, as John Lewis taught us, is essential in fighting injustice. But when protests became destructive abroad, authoritarian leaders seized the pretext and won the day, again and again.

This movie is the story of the Arab Spring nearly a decade ago, as brave people filled the streets to denounce dictators in Egypt, Libya, Syria, Bahrain and Yemen. Sometimes the dictators were toppled, but the protesters couldn’t consolidate their gains and speak with a clear voice. Ruthless leaders took advantage of the chaos and viciously suppressed dissent. Many ordinary people, exhausted by violence and disorder, cheered the men with guns.

If you watched the Arab Spring on the ground, as I did in some of those countries, it carried a harsh lesson: Street protests without a strategy for orderly, nonviolent change are usually doomed to fail. Anarchic, leaderless movements are lovable, but as the rock-throwers multiply, they bring disaster. The other side has more firepower.

This movie is the story of the activists in Hong Kong. Last September, I watched thousands of fearless men and women chanting their slogan: “Fight for freedom! Stand with Hong Kong!” As night approached on Harcourt Road, the front-line protesters with helmets and rocks and petrol bombs emerged to assault government offices. They looked very much like the rock-throwing front-liners you see in Portland.

Hong Kong felt like a glorious revolution, with cartoon heroes and villains, but it was obvious last year that unless the protesters could leverage their gains into a good political settlement, they would eventually lose to the dictators in Beijing.

President Trump on July 20 praised federal troops for arresting protesters in Portland, Ore., and said he would send more federal troops into other U.S. cities. (The Washington Post)

The Chinese crackdown, when it came in May, was surprisingly easy and relatively bloodless. Beijing waited until the protesters were fatigued and the enclave’s economy was vulnerable, and then imposed a harsh national security law.

Last September, I asked Martin Lee, for decades a leader of nonviolent protest in Hong Kong, where this story was heading. “How will it end? I don’t know,” he said. But Lee probably suspected what was ahead. He was imprisoned in April; when he emerged, he counseled compromise before it was too late. He was denounced by the hyper-activists and then, whump, the crunch came.

For President Trump and his campaign, the Portland protests and similar eruptions elsewhere are a dream come true. This is a president who, quite literally, has nothing to run on — no positive achievements to show. His only chance is that his adversaries will create opportunities for him. Let’s be honest: The television footage of police ducking rocks, bottles and other debris plays directly into Trump’s hands. A country that looks like it’s coming apart — “American carnage,” as Trump cynically called it during his 2017 inaugural speech — is the ruined turf on which he would like to fight for reelection in November.

Joe Biden’s challenge is to take this battleground away from Trump. He needs to show that as president, he could truly bring America together by uniting our racially and politically diverse nation and restoring law, order and justice — three essential pillars of our democracy.

Biden shouldn’t wade into the crowds in Portland. But perhaps he can become the voice of conciliation and political change. He should affirm the rightness of the protesters’ cause — that black lives matter. But he should also call out the violent rock-throwers among the thousands of peaceful demonstrators. Take the risk of offending an extreme wing of his base to gather broad support from the vast majority of Democrats whose overwhelming priority is to defeat Trump.

Biden’s job is to show that Trump, the supposed law-and-order candidate, is in fact the voice of illegality and disorder. The presumptive Democratic nominee, through his surrogates, should help mobilize the nonviolent majority in Portland. Trump’s thuggish federal agents can’t stop the nightly standoff at the federal courthouse. They make it worse. But maybe courageous progressives, by insisting on nonviolence and policing their own ranks, could de-escalate this confrontation before it further divides the country.

“The president has made clear that he wants conflict between protesters and police as a central theme of his campaign,” argued Rep. Jerrold Nadler (D-N.Y.) Tuesday in grilling Attorney General William P. Barr. Biden’s task is to embody the opposite approach, by demonstrating that he has the guts and moral principles to unify our angry, bruised America.

Now’s the time to start showing what a good president would do.

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