I have known Mr. Trump casually for many years, and we have always been on friendly terms. I even agreed to appear on “The Apprentice” -- twice. But he has run the most divisive and demagogic presidential campaign I can remember, preying on people’s prejudices and fears. Abraham Lincoln, the father of the Republican Party, appealed to our “better angels.” Trump appeals to our worst impulses.

Threatening to bar foreign Muslims from entering the country is a direct assault on two of the core values that gave rise to our nation: religious tolerance and the separation of church and state. Attacking and promising to deport millions of Mexicans, feigning ignorance of white supremacists, and threatening China and Japan with a trade war are all dangerously wrong, too. These moves would divide us at home and compromise our moral leadership around the world. The end result would be to embolden our enemies, threaten the security of our allies, and put our own men and women in uniform at greater risk.