Cedric L. Alexander ( @Calex_law ) is a past president of the National Organization of Black Law Enforcement Executives and CEO of CL Alexander Consulting LLC. He is the author of the forthcoming book, "In Defense of Public Service: How 22 Million Government Workers Will Save Our Republic" (Berrett-Koehler Publishers). The opinions expressed in this commentary are his own. Read more opinion articles on CNN.

(CNN) "I am the law and order candidate," GOP nominee Donald Trump proclaimed in 2016, days after a gunman killed five Dallas police officers on July 7, 2016. Many in law enforcement, from patrol officers on up to police executives, were gratified to hear that. And they should have been. For whatever else American presidents are and do, they are required by Section 3 of Article II of the Constitution to "take Care that the Laws be faithfully executed." Based on this, George Washington and Alexander Hamilton described the president as the nation's " chief magistrate ." And in July 2016, candidate Trump clearly acknowledged this most basic duty.

Tragically, President Trump has time and again failed to perform his Section 3 duty. He has repeatedly revealed himself as a lawless president, most recently in his tweets of July 14 directed against four freshmen Democratic congresswomen, Ilhan Omar of Minnesota, Alexandria Ocasio-Cortez of New York, Rashida Tlaib of Michigan and Ayanna Pressley of Massachusetts. His message to them, amplified in subsequent public statements and a July 17 re-election campaign rally in Greenville, North Carolina, was "go back" to the "totally broken and crime infested" places from you came.

Many people—in the media, in Congress and in the street—condemned all this as "classic" racism and xenophobia, an ugly echo of toxic cries once commonly hurled at Catholic, Jewish, Irish, Italian, Chinese, Japanese and many other immigrants or sons and daughters of immigrants. As a child of color growing up in the South, I sometimes heard "Go back to Africa" hurled at me and others. I never expected a sitting American president to echo it. Despite knee-jerk protestations from most of the GOP, it is inarguably right to call Trump's echo racist. No one should need me to tell them that.

Today's American communities are diverse, as are the families within those communities. The police are of these communities. Like the modern American military, today's police forces reflect the diversity of our communities and our families. If you tell recent immigrants or the sons and daughters of recent immigrants to go back, if you tell people of color to go back, if you tell Muslims to go back, a great many of the people you are wounding with your divisive words are American soldiers, sailors, marines, airmen and law enforcement officers. Every one of them has sworn to serve and protect, even at the cost of their own lives. And you are telling some of them—you don't even know who—to leave.

No one, especially an elected public servant, should need me to tell them this. But I have given 40 years to law enforcement, from deputy sheriff to police chief and director of public safety, and I do ask you to let me to tell you something else.

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In their ignorance and bigotry, the tweets and subsequent remarks of the 45th President, our top law enforcement officer, sworn (like every other law enforcement officer) to "preserve, protect and defend the Constitution," violate the essence of what our system of law represents. Among others, Senator Tim Kaine, Hillary Clinton's running mate in 2016, has noted that the Equal Employment Opportunity Commission (EEOC), an agency of the Executive Branch, overseen by the President, brands as illegal "ethnic slurs and other verbal or physical [workplace] conduct [based on] nationality," specifically citing "comments like, 'Go back to where you came from.'"

According to these EEOC regulations, our president's comments — if they were coming from anyone else in any other workplace in America -- would likely be unlawful. But he has also done far worse. He has not merely violated the law's spirit but denied it, abandoned it, renounced it, and, worst of all, failed even to understand it.

"Go back to where you came from" is historically the slur of a bigot, especially given that Trump's four targets are all US citizens, three by birth, one by naturalization. But telling any American, let alone duly elected members of Congress, that they should leave the country because they have criticized some aspect of it—that is the slur of a profoundly ignorant bigot. It reveals total ignorance and incomprehension of the Constitution and the First Amendment, which protects freedom of speech and, in particular, the right "to petition the Government for a redress of grievances."

In other words, we Americans have the absolute right to criticize our government. I would argue that, if we truly love our country, we have the affirmative responsibility to criticize it when necessary. Without question, elected members of our government (in contrast, for example, to Putin's government) have a duty never to slavishly uphold the status quo but to responsibly question and criticize it. After all, the Preamble to the Constitution makes no claim to having formed a perfect Union but speaks of working toward "a more perfect Union." That work does not end.

By "othering" people who do not share his myopic tunnel vision of America, by telling them that they do not belong, President Trump is dividing our house against itself. I know from long experience that policing is most effective when the officers feel connected to the community and the community feels connected to them. The best police leaders work every day to make "a more perfect Union" between police and community. They strive to instill in their officers a reverence for constitutional rights and an empathy for their fellow beings. They want bold, courageous officers, not authoritarian tough guys who see themselves as members of an army occupying hostile territory. Creating community-oriented police departments is hard work, and the last thing beat cops and their commanders need is for the nation's "chief magistrate" to make it infinitely harder.

When I was a police chief in Rochester, New York, and a director of public safety in DeKalb County in metro Atlanta, and when I had the high honor of serving on President Obama's Task Force on 21st Century Policing, I understood that, whatever specific duties were outlined in my job description, I was expected to be above all else a leader. I was expected to articulate standards of behavior and performance consistent with my conservative interpretation of my constitutional oath as a law enforcement officer. I was also expected to demonstrate these standards in everything I myself said and did. That was the job I had sworn to do. It wasn't always easy. But it was always clear.

Democracy demands leadership. Every police officer—city cop, county deputy, federal agent, FBI or ICE—needs the leadership direction and example of their commanders, including the "chief magistrate" of the United States. Democracy demands law and order. The faithful execution of law and order demands unbiased, thoroughly informed, completely comprehending, totally committed and deeply empathetic leadership.

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Go back, Mr. President, go back to the Constitution. Read it—or have it read aloud to you. Study Article II especially hard. Come back only when you finally comprehend it, are able to embrace it, and are willing to lead according to it. We need that from you. We need this from you, the chief magistrate, the top law enforcement executive in the nation. Unless you, Mr. President, in perfect harmony with the Constitution and the laws that flow from it, set an example of faithfully executing the laws without dividing the nation or alienating any of its racially and ethnically diverse communities, law enforcement will become an all but impossible enterprise in America. You have an opportunity and an obligation to support the police by both living and delivering the message that police and public form one nation, indivisible under law.