There is no recent precedent in American politics for such unrelenting and direct attacks on a presidential nominee’s commitment to the basic institutions of democracy. Alvin S. Felzenberg, a historian and conservative columnist, said the latest point of comparison for this week’s Democratic barrage might be the 1940 election, in which Republicans savaged Franklin D. Roosevelt for defying tradition to pursue a third term as president. Even that, Mr. Felzenberg said, was “not with this tenacity and not at the conventions per se.”

But Mr. Trump has stirred concerns throughout the 2016 race, on the left and right, with a political approach and a set of policy prescriptions that plainly defy the norms of American politics. He has campaigned on pledges to bar Muslims from entering the United States and to torture people suspected of terrorism. (Mr. Trump has recently suggested barring people from the United States based on their country of origin, though he has not ruled out a religious test.)…

Mr. Trump has routinely praised autocratic foreign leaders for what he characterizes as their steely leadership abilities. He has hailed Saddam Hussein, the executed Iraqi leader, for his skill at maintaining power, and said of Kim Jong-un, the North Korean leader, “You have to give him credit” for consolidating authority. This week, amid reports that Russian-backed hackers had breached the Democratic National Committee, Mr. Trump said he considered Mr. Putin a superior leader to Mr. Obama…

Mr. Manafort, the Trump campaign chairman, indicated that Mr. Trump would not back away from presenting himself as a rescuer for a nation in peril. “The reason Donald Trump’s speech at the convention worked was because he was saying what people are feeling and thinking,” Mr. Manafort said in an email.