...His latest book, “Friends Divided: John Adams and Thomas Jefferson, ” provides an illustration. The antagonism between Adams’s Federalists and Jefferson’s Republicans in the 1790s was far more fundamental, and therefore more threatening, than American partisanship today: “I think we’re going to survive easily,” Mr. Wood says.

By contrast, Adams, Jefferson and their coalitions came close to killing the republic in its cradle. They disagreed on as fundamental a question as whether the new republic should be democratic. Jefferson had a romantic faith in democracy and the wisdom of ordinary people; Adams predicted that “democracy will infallibly destroy all civilization.” 

Jefferson’s view was partly self-serving. “The leadership of the Republican Party, which is the popular party, is Southern slaveholders,” Mr. Wood says. “They don’t fear the people,” because the gentry-aristocracy effectively controlled electoral outcomes. Jefferson was akin to today’s “limousine liberal” in that he was insulated from the policies he promoted. (Eventually, his ideas would prove potent in arguing against slavery.) Meanwhile, Adams’s Federalists “are coming from New England, where you have far more egalitarian societies, far more democratic societies,” he says. “But for that very reason, the leaders are more scared of populism, of democracy.” 

That may make Adams sound like a member of today’s “establishment.” Yet some of his other ideas would be more amenable to populists like Donald Trump. Adams said to Jefferson, in Mr. Wood’s paraphrase: “You fear the ‘one’ of monarch, I fear the ‘few,’ meaning the aristocrats.” Adams argued that domination by oligarchs was a grave threat to liberty. “It’s his way of justifying the strong executive who will act as a check on the few,” Mr. Wood says. Adams wanted the executive to have some of the powers of the Crown.

That was anathema to Jefferson, whose life mission was “the elimination of monarchy, and all that it implies, which is hereditary rule, hierarchy and corruption.” He saw around him “a world of privilege in which ordinary people are abused. . . . From our point of view, he’s very sympathetic because he’s destroying that world,” Mr. Wood says....