Trump 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. 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.