Who could possibly imagine a repeat of Chicago 1968? In the build-up to that year’s Democratic convention Lyndon Johnson, the US president, dropped out of the race, assassins’ bullets killed Bobby Kennedy and Martin Luther King, and many of America’s inner cities went up in flames. The past is another country, as they say. Yet the closer we get to the 2016 Republican convention in Cleveland, the more its convulsive predecessor looms.

Donald Trump has made clear he would view any outcome to Cleveland other than his nomination as theft. Party insiders, however, make clear that under the rules, which they have yet to draw up, theft could magically become legal — indeed desirable. “I think you’d have riots,” said Mr Trump. Cleveland’s mayor has placed an order for 2,000 riot gear suits. It is hard to see how another nominee could be fixed on the quiet.

The core dispute is differing views on party democracy. Mr Trump will almost certainly head into Cleveland in July with the most delegates. He needs 1,237 to win outright. That prospect receded last week when Ted Cruz, his only real competitor, beat him easily in Wisconsin. Even if Mr Trump wins most of the re­maining primaries, including New York next week, he will be lucky to cross the magic line. The chances are that he will head into Cleveland with something short of a majority and that Mr Cruz will be some distance behind him.

Almost all delegates are “bound” to vote for their nominee on the first ballot. After that most are free to do what they like. Since Mr Trump is terrible at ground organisation, his delegates are far more likely to switch to Mr Cruz than vice versa. One of two things will then happen: enough votes will shift to Mr Cruz on the second ballot to make him the Republican nominee, or both will again fail to reach a majority. At that point, a white knight, such as John Kasich, the relatively moderate governor of Ohio (Cleveland’s state), or Paul Ryan, speaker of the House of Representatives, would emerge. Out of the ashes of Cleveland, Hillary Clinton would suddenly have a competitor.

That, at least, is what party insiders are plotting. Would it amount to a boardroom coup? If they secure at least three-quarters of the delegates between them, as looks likely, both Mr Trump and Mr Cruz would think so. Mr Trump’s people even talk of “treason”. It is possible they will swallow their pride and combine on one ticket, with Mr Cruz agreeing to be Mr Trump’s running mate. But Mr Cruz would have no power to instruct his delegates to vote for Mr Trump. Many of them would probably view a Trump nomination with as much horror as the rest of us.

More likely is that the two men would be unable to see past their mutual antipathy. The Trump-Cruz war of insults has long since breached politics’ normal limits. How could Mr Trump buy off someone he taunts as “Lying Ted”? How could Mr Cruz play second fiddle to someone who is “unfit to be president”.

This is where Chicago comes in. In his book, Miami and the Siege of Chicago, Norman Mailer described what happens when a party’s base severs from its leadership. Inside the embattled hall, Democratic party brokers fixed the nomination of Hubert Humphrey, the unpopular vice-president who backed the hated Vietnam war. Outside, mayor Richard Daley’s cops beat up the protesting hippies. “The Democratic Party had here broken into two before the eyes of a nation like Melville’s whale charging right out of the sea,” wrote Mailer. Of course, the content of the revolt was light years from today’s Republican insurgency. The protesters’ platform called for the abolition of money, disarmament of the police and full unemployment. “Let the machines do it,” read their petition.

Their demands were the polar opposite of what Mr Trump’s angry white male supporters want. Stop Mexicans from stealing our jobs, would be their rallying cry. If Mr Trump were denied the nomination, it is easy to imagine him directing that anger at the Republican elites. His close friend, Roger Stone, recently threatened to guide Trump supporters to anyone who switched their vote. “We will disclose the hotels and the room numbers of those delegates who are directly involved in the steal,” he said. It is easy to see why Republican leaders would do anything — even risk provoking violence — to deny their party’s crown to Mr Trump. Many would be equally loath to hand it to Mr Cruz.

In their eyes it is the Republican party, which is a private body, that selects its nominee — not the public. It is the party’s club. They make the rules. The ultimate battle will thus be over who sits on the Cleveland rules committee. Each state nominates one man and one woman to it. Thirty-one states have a Republican governor. Very few are fans of Mr Trump. Ditto for Mr Cruz. Each governor has a stake in minimising Republican losses in November. What are the odds they will push for rules that would kill Mr Trump’s chances after the first ballot? Reasonable and rising. What are the chances Mr Trump would accept this? Low and falling.

Mailer wrote in 1968 of party captains hovering “above a directionless void, there to loose the fearful nauseas of the century”. His prose defined the summer. It threatens to do so again.

edward.luce@ft.com