If in 2016 various factions of the electorate were prepared to look beyond Mr. Trump’s bombast in the hope that he might deliver whatever it was they wanted without too much damage to the nation, today there is no mystery about what he will do should he win, about the sorts of people he will surround himself with and the personal and political goals he will pursue. There is no mystery, either, about the consequences for the world if America re-elects a leader who openly displays his contempt for its allies.

Mr. Trump’s four years in the White House did lasting damage to the presidency and to the nation. He deepened existing divisions among Americans, leaving the country dangerously polarized; he so demeaned public discourse that many Americans have become inured to lies, insults and personal attacks at the highest levels of leadership. His contempt for the rule of law raised concerns about the long-term stability of American democracy, and his absence of a moral compass threatened to corrode the ideals of national service.

The Republic weathered Mr. Trump’s presidency for a variety of reasons: his lack of prepared agenda, the disruptions of the Covid-19 pandemic and the efforts of appointees who tried to temper his most dangerous or unreasonable demands. Most important, it survived because of the people and institutions in his administration and in the Republican Party who proved strong enough to stand up to his efforts to undermine the peaceful transfer of power.

It is instructive in the aftermath of that administration to listen to the judgments of some of these officials on the president they served. John Kelly, a chief of staff to Mr. Trump, called him the “most flawed person I’ve ever met,” someone who could not understand why Americans admired those who sacrificed their lives in combat. Bill Barr, who served as attorney general, and Mark Esper, a former defense secretary, both said Mr. Trump repeatedly put his own interests over those of the country. Even the most loyal and conservative of them all, Vice President Mike Pence, who made the stand that helped provoke Mr. Trump and his followers to insurrection on Jan. 6, 2021, saw through the man: “On that day, President Trump also demanded that I choose between him and the Constitution,” he said.

There will not be people like these in the White House should Mr. Trump be re-elected. The former president has no interest in being restrained, and he has surrounded himself with people who want to institutionalize the MAGA doctrine. According to reporting by the Times reporters Maggie Haberman, Charlie Savage and Jonathan Swan, Mr. Trump and his ideological allies have been planning for a second Trump term for many months already. Under the name Project 2025, one coalition of right-wing organizations has produced a thick handbook and recruited thousands of potential appointees in preparation for an all-out assault on the structures of American government and the democratic institutions that acted as checks on Mr. Trump’s power.

The project ties in with plans from Mr. Trump and his supporters to reclassify tens of thousands of federal workers so they can be fired if they do not buy fully into the Trump agenda. He also plans to strip the Justice Department of its independence in order to use it to wreak vengeance on those who, in his view, failed to concoct a victory for him in the 2020 election or otherwise didn’t support his unconstitutional demands. There is more, including threats by Mr. Trump to find ways to use federal troops against those who might protest his policies and practices. These ambitions demonstrate that the years out of office and the mounting legal challenges he faces have only sharpened his worst instincts.

Mr. Trump was impeached twice as president and since leaving office has been charged in four criminal cases — two related to his efforts to overturn the 2020 election, one over hush money paid to a porn star and another for hoarding classified documents after he left office and impeding the government’s efforts to retrieve them. No other sitting or former president has ever been indicted on criminal charges. Not only has Mr. Trump shown no remorse for these actions, he has given no sign that he understands these indictments to be anything but a political crusade meant to undermine him. He continues to claim that the Jan. 6 insurrection has been misrepresented. “There was love and unity,” he said in an interview last August. And he has suggested that, if re-elected, he could use his presidential powers to pardon himself.

Mr. Trump’s forays into foreign affairs remain dangerously misguided and incoherent. During his presidency, he displayed consistent admiration for autocratic leaders — including Xi Jinping, Vladimir Putin and Kim Jong-un — and contempt for our democratic allies. While in the White House, he repeatedly threatened to leave NATO, an alliance critical to the stability of Europe that he sees only as a drain on American resources; now his campaign website says, without elaborating, that he plans to “finish” the process of “fundamentally re-evaluating NATO’s purpose and NATO’s mission.”

He has announced his intention to abandon Ukraine, leaving it and its neighbors vulnerable to further Russian aggression. Encouraged by an American president, leaders who rule with an iron fist in Hungary, Israel, India and elsewhere would face far less moral or democratic pressure.

Mr. Trump has made clear his conviction that only “losers” accept legal, institutional or even constitutional constraints. He has promised vengeance against his political opponents, whom he has called “vermin” and threatened with execution. This is particularly disturbing at a time of heightened concern about political violence, with threats increasing against elected officials of both parties.

He has repeatedly demonstrated a deep disdain for the First Amendment and the basic principles of democracy, chief among them the right to freely express peaceful dissent from those in power without fear of retaliation, and he has made no secret of his readiness to expand the powers of the presidency, including the deployment of the military and the Justice Department, to have his way.

Democracy in the United States is stronger with a formidable conservative political movement to keep diversity of thought alive on important questions, such as the nation’s approaches to immigration, education, national security and fiscal responsibility. There should be room for real disagreement on any of these topics and many more — and there is a long tradition of it across the American experiment. But that is not what the former president is seeking.

Re-electing Mr. Trump would present serious dangers to our Republic and to the world. This is a time not to sit out but instead to re-engage. We appeal to Americans to set aside their political differences, grievances and party affiliations and to contemplate — as families, as parishes, as councils and clubs and as individuals — the real magnitude of the choice they will make in November.

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The editorial board is a group of opinion journalists whose views are informed by expertise, research, debate and certain longstanding values. It is separate from the newsroom.