In a recent radio interview, Donald Trump said that “the saddest thing is, because I am the president of the United States, I am not supposed to be involved with the Justice Department. I’m not supposed to be involved with the FBI. I’m not supposed to be doing the kind of things I would love to be doing ...”

Trump then asked, referring to the Justice Department and the FBI, “Why aren’t they going after Hillary Clinton with her emails and with her dossier ...?”

In a series of tweets the next morning, Trump called on the Justice Department and the FBI to “do what is right and proper” by launching criminal probes of Clinton.

Trump’s obvious aim was to deflect attention from Special Counsel Robert Mueller’s probe of his campaign, and of the indictments issued against his former campaign aides. But by calling on the Justice Department to investigate Clinton, and by lamenting that he cannot do “the kind of things I would love to be doing,” Trump crossed a particularly dangerous line.

In a democracy bound by the rule of law, presidents do not prosecute their political opponents. Nor, until now, have they tried to stir up public anger toward their former opponents.

Our democratic system of government depends on presidents putting that system above their own partisan aims.

As Harvard political scientist Archon Fung has noted, once an election is over, candidates’ graciousness to one another is an important demonstration of their commitment to the democratic system over the specific outcomes they fought to achieve. This helps re-establish civility and social cohesion. It reminds the public that our allegiance is not toward a particular person or party but to our system of government.

Think of Al Gore’s concession speech to George W. Bush in 2000, after five weeks of a bitterly contested election and just one day after the Supreme Court ruled 5-4 in favor of Bush. “I say to President-elect Bush that what remains of partisan rancor must now be put aside, and may God bless his stewardship of this country.”

Gore publicly bowed to the institutions of our democracy. “Now the U.S. Supreme Court has spoken,” he said. “Let there be no doubt, while I strongly disagree with the court’s decision, I accept it. ... And tonight, for the sake of our unity as a people and the strength of our democracy, I offer my concession.”

Bush’s response to Gore was no less gracious.

“Vice President Gore and I put our hearts and hopes into our campaigns; we both gave it our all,” Bush said. “We shared similar emotions. I understand how difficult this moment must be for Vice President Gore and his family. He has a distinguished record of service to our country as a congressman, a senator and a vice president.”

Many voters continued to doubt the legitimacy of Bush’s victory, but there was no social unrest, no civil war. Americans didn’t retreat into warring tribes.

Think of what might have occurred if Gore had bitterly accused Bush of winning fraudulently and blamed five of the Republican appointees on the Supreme Court for siding with Bush for partisan reasons.

Think of what might have happened if, during his campaign, Bush had vowed to put Gore in jail for various improprieties, and then, after he won, called on the Justice Department and the FBI to launch a criminal investigation of Gore.

Such statements — close to ones that Trump has actually made — might have imperiled the political stability of the nation.

Instead, Gore and Bush made the same moral choice their predecessors made after every previous American presidential election, and for the same reason.

They understood that the demonstrations of respect for each other and for the Constitution confirmed the nation’s commitment to our system of government. This was far more important than their own losses or wins.

Donald Trump has no such concern.

This is the essence of Trump’s failure as president — not that he has chosen one set of policies over another, or that he has lied repeatedly and chronically, or even that he has behaved in childish and vindictive ways unbecoming a president.

It is that he has sacrificed the processes and institutions of American democracy to achieve his own selfish ends.

By saying and doing whatever he believes it takes for him to come out on top, Trump has abused the trust we place in a president to preserve and protect the nation’s capacity for self-government.

This will be his most damaging and most damning legacy.

© 2017 Robert Reich

Robert Reich, a former U.S. secretary of labor, is a professor of public policy at UC Berkeley. He blogs daily at www.facebook.com/rbreich. To comment, submit your letter to the editor at SFChronicle.com/letters.