Even those who have grown used to Mr. Trump’s conduct in office may have found themselves newly alarmed by the grim spectacle of a sitting president deliberately stoking the country’s divisions and pursuing personal vendettas in the midst of a crisis that has Americans fearing for their lives and livelihoods.

Since well before he became president, Mr. Trump’s appetite for conflict has defined him as a public figure. But in recent days he has practiced that approach with new intensity, signaling both the depths of his election-year distress and his determination to blast open a path to a second term, even at the cost of further riling a country in deep anguish…

Still, Mr. Trump’s behavior has rattled even some supportive Republicans, who believe it is likely to backfire and possibly cost them the Senate as well as the White House. It has also further alarmed Democrats, who have long warned that Mr. Trump would be willing to use every lever of presidential power and deploy even the most unscrupulous campaign tactics to capture a second term.