IT HAD not seemed possible, but Donald Trump descended this week to a new low of bigotry, fear-mongering and conspiracy-peddling. Republican leaders who said last week that they expected a change in tone after Mr. Trump’s racist attacks on a California judge quickly received their answer. What can House Speaker Paul D. Ryan (R-Wis.) or Senate Majority Leader Mitch McConnell (R-Ky.) possibly say now? As the country mourned the wanton slaughter of 49 people early Sunday, the presumptive Republican presidential nominee took a victory lap, hinted darkly that President Obama is an enemy of the nation, libeled American Muslims and, in grotesque punctuation, finished up with a vindictive attack on the media .

“Appreciate the congrats for being right on radical Islamic terrorism,” he tweeted. “I don’t want congrats,” he continued, as though that were not exactly what he wanted, “I want toughness & vigilance.” Mr. Trump may have calculated that a suddenly anxious electorate would be more receptive to his campaign of fear and prejudice, emotions he immediately attempted to inflame.

In a Monday speech, Mr. Trump painted a false picture of a nation infiltrated by waves of unscreened Muslim refugees and immigrants, who, abetted by Democrats, are destroying American values and threatening the public. Among other things, he chillingly accused Muslim Americans of complicity with terrorists: “The Muslims have to work with us,” he said. “They know what’s going on. They know that [the Orlando shooter] was bad. They knew the people in San Bernardino were bad. But you know what? They didn’t turn them in. And you know what? We had death, and destruction.”

The following shouldn’t have to be repeated, but Mr. Trump makes it necessary. Most American Muslims are as patriotic and law-abiding as most American Christians, Jews and Hindus. Many have fought for and are fighting for the United States in dangerous theaters far away. To generalize as Mr. Trump does about “the Muslims” is to set the nation down a dangerous road it has trod, to its eventual regret, in the past: banning Chinese immigrants a century ago, rounding up U.S. citizens and noncitizens of Japanese descent in the 1940s, expelling “wetbacks” a decade later.

Speaking in Manchester, N.H. June 13, Republican presidential candidate Donald Trump vowed to shut down immigration from countries "where there is a proven history of terrorism against the United States." Here are key moments from that speech. (Associated Press/Reuters)

Mr. Trump also raised suspicion in television interviews that Mr. Obama wants terrorists to strike the United States, or at least looks the other way as they scheme. “We’re led by a man that either is not tough, not smart or has something else in mind. And the something else in mind — people can’t believe it. People cannot believe that President Obama is acting the way he acts and can’t even mention the words ‘radical Islamic terrorism.’ There’s something going on.” He invited poisonous speculations about his Democratic opponent’s motives, as well: Hillary Clinton “wants to allow radical Islamic terrorists to pour into our country,” he said. Informal Trump adviser Roger Stone, meanwhile, claimed that Huma Abedin, a top Clinton aide, might be “a terrorist agent.”

Mr. Trump capped a day of assaulting fundamental liberal democratic values by announcing he would ban Post reporters from covering his campaign events. If this is his inclination now, imagine how he might wield the powers of the presidency.

Before the Orlando shooting, Beltway analysts speculated about how a terrorist attack might affect the presidential election. Now we know at least part of the answer: Mr. Trump would reveal himself more clearly than ever as a man unfit to lead.