Within the space of a few months, the prospect of a contested convention in the United States has gone from outlandish to the Republican Party establishment’s last, best hope of stopping businessman Donald Trump.

Before voting started, this website argued that an open convention was unlikely. Reporters dream of one every four years, but every four years the parties manage to find a nominee in time.

For good reason. The last time the Democrats needed to “broker” their convention was in 1952. The last time the Republicans had one was in 1948. At both times, the parties went on to lose the general election. The spectacle of a party struggling to agree on a presidential nominee doesn’t inspire much confidence in voters.

Design

Parties have designed the nominating process in such a way as to avoid a contested convention. They have written caucus and primary rules that keep hopeless contenders out. A few early states are supposed to winnow the field before the big bang of Super Tuesday eliminates the remaining also-rans.

That’s the theory anyway.

It has gone to plan on the Democratic side where former secretary of state Hillary Clinton has the nomination all but wrapped up.

But this year’s Republican contest is breaking all the rules.

Four-way split

By the Associated Press’ count, Trump is a third of the way to the nomination with 384 of the 1,237 delegates he would need to win outright.

If he were any other candidate, the New Yorker would probably be considered the presumptive nominee at this point and others would be dropping out of the race.

But Trump is not any other candidate and his competitors are not giving up.

Texas senator Ted Cruz, who has 300 delegates, still has hope of defeating Trump one-on-one, but his best chance was probably in the South and most states there have voted.

For Florida senator Marco Rubio, who is at 151 delegates, and Ohio governor John Kasich, who has just 37 pledged to him so far, the only hope is a contested convention.

Decisive

The winner-takes-all contests in Florida and Ohio next week could prove decisive.

If Trump wins them both, which seems unlikely, he would only need half the remaining delegates to clinch the nomination. That would be doable, especially if Rubio and Kasich drop out after losing their homes states.

But if Trump loses in both states, he would need to win two-thirds of the remaining delegates to win. That is almost impossible.

Some of the bigger states that vote late in the Republican contest, including New York, Oregon and Pennsylvania, favor Democrats in the general election and are unlikely to give Trump more than the 35 to 40 percent support he has garnered elsewhere.

Those states also allocate their delegates on a proportionate rather than a winner-takes-all basis, meaning Trump could not walk away with all of them by winning only a plurality of the votes as he did in South Carolina last month.

Floor fight

Trump and his supporters would be outraged if the convention denied him the nomination this summer even if he had only, say, 40 percent of the delegates. But if the party really wanted to, it shouldn’t be too hard to nominate someone else.

By far most of the delegates pledged to Trump would be unbound on a second ballot, meaning they can vote for anybody.

Delegates are a combination of local and state party officials and volunteers. The former are unlikely to support Trump even if they must vote for him on the first ballot. Once he has failed to win a majority, though, they will be looking to make a deal.

Delegates aren’t bound for any other business than the presidential nomination. They can nominate any vice presidential candidate they want and have total autonomy when it comes to voting on the party platform and the convention rules.

Michael Nelson writes at The Cook Political Report that a floor fight over Republican Party Rule 40(b) could be the start of an effort to stop Trump.

That rule requires that a nominee has the support of a majority of the delegates from at least eight states or territories. Given that no candidate might meet the threshold, the convention would need to overturn the rule first and that could be the basis for an alliance among Trump’s opponents.

Delegates are free to vote as they wish on rules or platform challenges regardless of which candidate they are required to support on the first ballot. If Trump were to be on the losing side of one or more such challenges, his momentum would be slowed if not checked.

It has taken more than half a century, but political reporters could finally get their wish.