Politicians praise America’s founders for having set guiding and enduring principles. Donald J. Trump declares that global free trade “is a direct affront to our founding fathers, who wanted America to be strong, independent and free.” Hillary Clinton counters, “Our founders embraced the enduring truth that we are stronger together.”

But that raises questions: Which founders, and which principles? For in history, unlike in mythic memory, they fought like cats and dogs over every major issue, foreign and domestic.

Thomas Jefferson’s followers called themselves Republicans, but their enemies called them Democrats — just to confuse us today. They battled the Federalists, led by Alexander Hamilton, of later musical fame. Where Jefferson wanted to construct the Constitution narrowly and favored a decentralized country with a weak federal government, Hamilton and his allies favored a broad interpretation, with a powerful, centralized state that promoted economic development and exercised global power.

Instead of offering a single, cohesive and enduring vision for America, the founders were diverse and squabbling. They generated contradictory political principles that persist to our own day. Instead of offering us an antidote to our divisions, those clashing founders created them.

Our early politics were so edgy and shrill because the stakes involved were so high, as leaders and their followers struggled to define the revolution and Constitution. The union of states and the republican form of government were new, tenuous, vulnerable and open to debate. It was easy to imagine one’s political rivals as ominous threats to free government. When Mr. Trump accuses Mrs. Clinton of cofounding the Islamic State, he echoes the recklessness with which Hamilton associated Jefferson with the bloody Jacobins of the French Revolution. ...