WHEN PRESIDENT TRUMP attacked Attorney General Jeff Sessions in a tweet Tuesday for not aggressively investigating Hillary Clinton, most attention focused, understandably, on the implications for Mr. Sessions. Yet even more alarming than the president’s assault on his own attorney general is Mr. Trump’s return to the “lock her up” theme of his 2016 campaign. We need to recall, once again, what it means to live under the rule of law. Since his inauguration six months ago, so many comparisons have been made to “banana republics” that it is almost unfair to bananas. But there is a serious point to be made about the difference between the United States of America and a state ruled by personal whim.

In a rule-of-law state, government’s awesome powers to police, prosecute and imprison are wielded impartially, with restraint and according to clearly defined rules. These rules apply equally to rich and poor, powerful and weak, ruling party and opposition. In such states, individuals advance on the basis of their talent and initiative, not whom they know. Companies invest where they think the returns will be highest, not to please those in power. The result is that, over time, rule-of-law states prosper. Banana republics do not.

No country ever has attained perfection in this regard, but the United States has been the envy of the world because certain norms have been accepted. After hard-fought elections, the losing side concedes and the winning side leaves the loser in peace to fight another day. Leaders are expected to speak truthfully to their citizens. They respect the essential nonpartisan nature of law enforcement and the military and key civic organizations such as the Boy Scouts of America. They show respect, too, for the political opposition.

[Trump’s latest rage tweets reveal a lawless, out-of-control president]

To list those basic expectations is to understand how low Mr. Trump is bringing his office. Just in the past few days, he urged Navy men and women to call Congress on behalf of his political goals and turned the National Scout Jamboree into an unseemly political rally, calling the nation’s politics a “cesspool” and a “sewer” and disparaging his predecessor and the media. Routinely he trades in untruths, even after they have been exposed and disproved. He has launched an unprecedented rhetorical assault on the independence of the Justice Department, the FBI and the special counsel’s office — and now he is again threatening his defeated 2016 opponent.

(Victoria Walker/The Washington Post)

Members of Congress who are, properly, investigating Russia’s interference in the 2016 race have not questioned Mr. Trump’s legitimacy. Ms. Clinton herself graciously conceded. The FBI thoroughly investigated her email practices and found no basis to prosecute. Yet Mr. Trump attacks Mr. Sessions for taking “a VERY weak position on Hillary Clinton crimes,” implying that a politically inspired reinvestigation might help the attorney general keep his job. It is disgusting.

Timidly, belatedly, but encouragingly, members of Mr. Trump’s party are beginning to push back. Last week, Rep. Michael McCaul, a Texas Republican who chairs the Homeland Security Committee, told NBC’s Andrea Mitchell that there would be “a tremendous backlash” from Republicans as well as Democrats if Mr. Trump attempted to fire special counsel Robert S. Mueller III, who is investigating Russia’s behavior in 2016 and any possible Trump campaign involvement. On Monday, House Speaker Paul D. Ryan (Wis.) also came to the counsel’s defense. “I don’t think many people are saying Bob Mueller is a person who is a biased partisan,” Mr. Ryan said. “He’s really sort of anything but.”

What’s at stake is much more than the careers of a particular attorney general or special counsel. The United States has been a role model for the world, and a source of pride for Americans, because it has strived to implement the law fairly. When he attacks that process and seeks revenge on his opponents, Mr. Trump betrays bedrock American values. It’s crucial that other political leaders say so.