This editorial in the Washington Post last weekend, entitled Donald Trump is a unique threat to American democracy, is the single best (among hundreds) of articles I've read about why Trump must never become President. Excerpt:

DONALD J. TRUMP, until now a Republican problem, this week became a challenge the nation must confront and overcome. The real estate tycoon is uniquely unqualified to serve as president, in experience and temperament. He is mounting a campaign of snarl and sneer, not substance. To the extent he has views, they are wrong in their diagnosis of America's problems and dangerous in their proposed solutions. Mr. Trump's politics of denigration and division could strain the bonds that have held a diverse nation together. His contempt for constitutional norms might reveal the nation's two-century-old experiment in checks and balances to be more fragile than we knew.

Any one of these characteristics would be disqualifying; together, they make Mr. Trump a peril. We recognize that this is not the usual moment to make such a statement. In an ordinary election year, we would acknowledge the Republican nominee, move on to the Democratic convention and spend the following months, like other voters, evaluating the candidates' performance in debates, on the stump and in position papers. This year we will follow the campaign as always, offering honest views on all the candidates. But we cannot salute the Republican nominee or pretend that we might endorse him this fall. A Trump presidency would be dangerous for the nation and the world.

…there is nothing on Mr. Trump's résumé to suggest he could function successfully in Washington. He was staked in the family business by a well-to-do father and has pursued a career marked by some real estate successes, some failures and repeated episodes of saving his own hide while harming people who trusted him. Given his continuing refusal to release his tax returns, breaking with a long bipartisan tradition, it is only reasonable to assume there are aspects of his record even more discreditable than what we know.

The lack of experience might be overcome if Mr. Trump saw it as a handicap worth overcoming. But he displays no curiosity, reads no books and appears to believe he needs no advice. In fact, what makes Mr. Trump so unusual is his combination of extreme neediness and unbridled arrogance. He is desperate for affirmation but contemptuous of other views. He also is contemptuous of fact. Throughout the campaign, he has unspooled one lie after another — that Muslims in New Jersey celebrated after 9/11, that his tax-cut plan would not worsen the deficit, that he opposed the Iraq War before it started — and when confronted with contrary evidence, he simply repeats the lie. It is impossible to know whether he convinces himself of his own untruths or knows that he is wrong and does not care. It is also difficult to know which trait would be more frightening in a commander in chief.

Given his ignorance, it is perhaps not surprising that Mr. Trump offers no coherence when it comes to policy. In years past, he supported immigration reform, gun control and legal abortion; as candidate, he became a hard-line opponent of all three. Even in the course of the campaign, he has flip-flopped on issues such as whether Muslims should be banned from entering the United States and whether women who have abortions https://www.washingtonpost.com/news/the-fix/wp/2016/03/30/donald-trump-there-has-to-be-some-form-of-punishment-for-women-who-get-abortions/. Worse than the flip-flops is the absence of any substance in his agenda. Existing trade deals are "stupid," but Mr. Trump does not say how they could be improved. The Islamic State must be destroyed, but the candidate offers no strategy for doing so. Eleven million undocumented immigrants must be deported, but Mr. Trump does not tell us how he would accomplish this legally or practically.

What the candidate does offer is a series of prejudices and gut feelings, most of them erroneous. Allies are taking advantage of the United States. Immigrants are committing crimes and stealing jobs. Muslims hate America. In fact, Japan and South Korea are major contributors to an alliance that has preserved a peace of enormous benefit to Americans. Immigrants commit fewer crimesthan native-born Americans and take jobs that no one else will. Muslims are the primary victims of Islamist terrorism, and Muslim Americans, including thousands who have served in the military, are as patriotic as anyone else.