Donald J. Trump has astounded the world, as he would be the first to tell you, leaving the Republican leadership to make the best of a terrible situation. Only months ago, Senator John McCain of Arizona, a professed maverick Republican, was berating Mr. Trump for inciting the fringe “crazies.” Now he backs the presumptive nominee. “I believe that the Republican Party must maintain its viability as a party,” he said.

But what if the Republicans are no longer a viable national party? What if the schism between Mr. Trump and establishment holdouts like Mitt Romney deepens, and other schisms follow? What if Mr. Trump’s achievement turns out to be not just hijacking the party of Ronald Reagan, but catalyzing its disintegration?

It might seem unthinkable, but we’ve been here before. Major-party crackups are rare in American history, but far from unprecedented. These convulsive events mark major phases in the long history of American democracy and its expansion.

And although none of the implosions are identical, they share elements with the current crisis inside the G.O.P., including nativism, political legitimacy and class. All of them resound with calls to make America great again, to reclaim a country that is on the verge of being stolen away.

The first momentous collapse occurred in the election of 1800, and nativism proved central to it. In order to suppress rising unruly democratic forces aligned with Thomas Jefferson, the dominant Federalist Party of John Adams and Alexander Hamilton stirred popular fears of a menacing enemy within, including immigrants friendly to the Jeffersonians. ...