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.”