Editor’s Note: Michael D’Antonio is the author of the book “Never Enough: Donald Trump and the Pursuit of Success” and co-author with Peter Eisner of “The Shadow President: The Truth About Mike Pence.” The opinions expressed in this commentary are those of the author. View more opinion articles on CNN.

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In the umpteenth scandalous moment of his reign, the President tweeted that four Congresswomen – three homegrown Americans and one who became a citizen of the United States at age 17 – should “go back and help fix the totally broken and crime infested places from which they came.” The racism in this attack is appalling, but it is not the only form of deviance on display.

That would be the suggestion, which he later amplified, that Donald Trump has anointed himself the judge of who belongs in America and who does not.

This should strike fear in every heart because it means that the most powerful person in the country – with a large cohort of supporters ready to follow him anywhere – has begun to sort and separate us from each other. In this moment he finds these four elected public servants unacceptable, but who’s to say what he’ll think about any of us tomorrow, should we dare to disagree with or challenge him?

This is a serious question, with terrifying implications, that voters need to examine before they cast their vote in the 2020 election.

Consider that as social beings who depend on our families, our communities, and yes, our country for our identity and well-being, few things are more upsetting than the prospect of being rejected and cast out. Shunning is so painful that most of us will go to great lengths to avoid it. Religious groups that use the threat of excommunication to keep people in line – “in the fold” –understand this powerful dynamic and so do bullies who make a show of victimizing one kid in order to dominate everyone on the playground.

As they make an example out of a target – a kid whose pain is quite obvious to other kids – schoolyard bullies can dominate everyone else in the schoolyard. If you didn’t witness this as a youngster, you can consult many studies that confirm the process. Later in life, bullies may use this tactic as bosses in a workplace or as coaches of teams (think Bobby Knight) or in politics. These bullies instill in others the fear of being fired, cut from a team, or defeated in an election. In this way they gain control.

No one is more conscious of the effects of fear than this President. When Bob Woodward asked Trump to reflect on the nature of power he said, “Real power is, I don’t even want to use the word, fear.” Indeed, in light of this confession, and everything else he learned about Trump’s White House, Woodward titled his book about the presidency “Fear.”

Where did Trump acquire his appreciation for the power of fear and his bullying ways? As his biographer, I locate the key moment in the summer of his thirteenth year, when his exasperated parents notified him that he was being sent away to military school because he was too unruly. Cast out of the family home, he did become more disciplined, but he also became the man who would tell me he enjoyed “all types of fights, even physical.”

As an adult, whether he was feuding with Mayor Ed Koch in the 1980s or attacking comedian Rosie O’Donnell and others more recently, Trump has regularly demonstrated his willingness to engage in public conflicts as a kind of sport. In business he readily filed lawsuits or defied those who would hold him to contracts. “When someone attacks me, I always attack back … except 100x more,” he boasted from the safety of Twitter in 2012. “This has nothing to do with a tirade but rather, a way of life!” These threats and displays of aggression showed that he wasn’t like other people. They also served as warning to those who might oppose him.

For a sense of how effective the bully’s posturing can be, consider the examples of Republican Senators Lindsay Graham and Ted Cruz. Both men ran for the GOP presidential nomination against Trump. In that race Graham said Trump was a “race baiting, xenophobic, religious bigot.” Cruz called him a “pathological liar” and mused that there was something “fundamentally wrong” with evangelical Christians who supported him.

Defeated in the primary elections and faced with the reality of Trump’s dominance of the Republican Party, Graham and Cruz became his acolytes. Amid the recent “go back” controversy Graham has refused to condemn Trump’s racism and Cruz has only noted the “heated rhetoric.”

In their silence, Cruz, Graham and others who won’t stand up to the bully are like children on the schoolyard who tremble at the thought of being separated from the group, and so throw in their lot with the bully. Uncertain if others would join them in standing up for what’s right, they give in to the fear of being next on Trump’s hit list.

The fear of being targeted, excluded – or even sent away, like the President was as a youngster – may also lurk in the hearts of voters who accept Trump’s behavior. With his attacks on the four members of Congress, his coldhearted crackdown on America’s immigrant families and asylum seekers, and his repeated effort to demonize those who disagree with him, Trump has demonstrated what happens if you step out of line.

The silence of lawmakers and others who won’t speak up ought to outrage – even frighten – those who hold dear the values and the promise of America.

But mute passivity is not the worst possible outcome here. Bullying and other threatening and rude behaviors can be contagious. We saw this in violent incidents at Trump rallies held before he was elected. One hates to imagine what will happen now that he is President – particularly if voters embolden him with a second term, making him more powerful than ever.