In many ways, this presidential election has been nothing short of historic. A candidate running openly as a socialist. A candidate who wasn’t born in the United States.

But the most enigmatic occurrence this year has been Donald Trump. Despite having no political experience, Mr. Trump leads the delegate race by a fairly wide margin—but is it enough of a margin?

Another way this election cycle could end up being historic is in the case of a brokered convention, which will happen if no single candidate gets the majority of delegates. A brokered Republican convention is becoming increasingly likely, and as Ben Ginsberg, a veteran Republican campaign lawyer, states, is “more possible than at any time in the modern era.”

How likely is it that a brokered convention will take place? Experts agree that whatever Trump’s final delegate count will be before the convention, it will be very close to the 1,237 he needs to get the nomination.

Although there are many possibilities when it comes to dividing the remainder of the delegates among the candidates, I’ll proceed with a brief description of a consensus of experts as published originally by FiveThirtyEight.com. Basically, June 7 is the key date.

Averaging expert opinions gives Trump 1,028 delegates by June 7, with five states left to go at that point. The most important of these is California, a winner-take-all state with 172 delegates total.

All other states with primaries or caucuses that day either won’t be voting for Trump (by a consensus of experts) or simply don’t have that many delegates. If Trump were to win California, he would have 1,200 delegates.

Although this is under the necessary amount of delegates, it’s important to keep in mind that there are 112 unbound delegates from places with no primaries or caucuses such as Colorado or American Samoa. We have no way of knowing how they will vote before their names are called on the convention floor. If even a fraction of them voted for Trump, they—along with any other delegates he gets from New Mexico—would push Trump over the 1,237 line, giving him the nomination.

Going one level deeper, how likely is it that Trump will win California? It depends on what you think are important factors in predicting elections. Current polling data provided by Real Clear Politics shows Trump with a consistent lead over rival Ted Cruz of about 8 points.

However, FiveThirtyEight weighted those polls with endorsements that Trump and Cruz have received. Basically, based on the principle that as we get closer to an election, voters listen more to party elites (as outlined by Georgetown Professor Hans Noel’s revolutionary 2008 book, The Party Decides), FiveThirtyEight has predicted what polls will look like closer to the election.

With those predictive polls in mind, FiveThirtyEight gives Cruz a 61 percent chance of winning California—thereby denying Trump the delegates he would need to clinch the party nomination, thus leading to a brokered convention.

A clear procedure has been laid out for the way in which the delegate voting at the Republican National Convention will take place if no candidate reaches a majority. Rule 40e of the Rules of the Republican Party states that “If no candidate shall have received [a] majority, the chairman of the convention shall direct the roll of the states be called again and shall repeat the calling of the roll until a candidate shall have received a majority of the votes entitled to be cast in the convention.”

The difference is that with each new round of voting, more delegates that were previously bound will become unbound. The exact number is extraordinarily hard to calculate, as it is a combination of state and party rules, but the New York Times estimates the number of unbound delegates to be about 5 percent in the first round of voting, 51 percent in the second round, and 80 percent in the third round.

At this point, all hell will break loose. Party bosses will quickly try to whip votes for the candidate favored by the establishment, who in this case is probably John Kasich, the only Republican left in the race who hasn’t run on an anti-establishment platform. Even write-ins are allowed, and there is a small possibility that someone who didn’t even run will be nominated.

All of this being said, a brokered convention is incredibly hard to predict with any certainty. But as many Republicans and many, many more Democrats seek any way to prevent Trump from getting the nomination, this looks more and more likely to be our best shot.