At the outset of this election year, with Donald Trump leading the race to be the Republican presidential nominee, Americans should pause to consider what a second Trump term would mean for our country and the world and to weigh the serious responsibility this election places on their shoulders.

By now, most American voters should have no illusions about who Mr. Trump is. During his many years as a real estate developer and a television personality, then as president and as a dominant figure in the Republican Party, Mr. Trump demonstrated a character and temperament that render him utterly unfit for high office.

As president, he wielded power carelessly and often cruelly and put his ego and his personal needs above the interests of his country. Now, as he campaigns again, his worst impulses remain as strong as ever — encouraging violence and lawlessness, exploiting fear and hate for political gain, undermining the rule of law and the Constitution, applauding dictators — and are escalating as he tries to regain power. He plots retribution, intent on eluding the institutional, legal and bureaucratic restraints that put limits on him in his first term.

Our purpose at the start of the new year, therefore, is to sound a warning.

Mr. Trump does not offer voters anything resembling a normal option of Republican or Democrat, conservative or liberal, big government or small. He confronts America with a far more fateful choice: between the continuance of the United States as a nation dedicated to “the blessings of liberty to ourselves and our posterity” and a man who has proudly shown open disdain for the law and the protections and ideals of the Constitution.

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.