With more than half the states having now held their nominating contests, Donald Trump and Senator Ted Cruz are quietly directing their attention to a second, shadow election campaign – one that is out of sight, little understood, but absolutely critical if Republicans arrive at their national convention with Trump short of a majority of delegates.
This parallel campaign is to select the individual delegates who will go to Cleveland in July for what could be the first contested convention in American politics in more than 60 years. Chosen through a byzantine process in each state, most of the delegates will become free agents if no one wins a majority on the first ballot.
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Analyst: Trump still not guaranteed nomination
Despite Donald Trump's wins in the primaries, he is still not guaranteed his party's nomination says Bill Galston of the Brookings Institution.
The mere prospect that delegates could deny Mr Trump the nomination led him to predict Wednesday that violence could erupt in such a situation.
"I think you'd have riots," Mr Trump warned. It seemed no idle speculation after the mayhem at Trump rallies. Speaking to CNN, Mr Trump said he still expected to reach the majority of 1237 delegates needed for a first-ballot nomination before the end of the nominating season. "I'm a closer," he said. "I get things closed."
For Mr Trump, the problem is that a delegate may be pledged to back one candidate, based on the results of a primary or caucus, but may be loyal to another. So Mr Trump could see the tide turn to a rival's favour on second and third ballots if he fails to hold the allegiance of delegates pledged to him.
Governor John Kasich's victory in his home state of Ohio on Tuesday increased the likelihood – if not the certainty – of a contested convention, highlighting the importance of how delegates are chosen.
Recruiting loyalists to run for delegate slots – often through a series of contests beginning at the precinct and county levels – favours campaigns with strong grass-roots networks and robust national organisations. Mr Trump has been lacking in both areas, failing to win in caucus states like Iowa, Kansas and Maine where a ground game is important.
"In the vast majority of the states, you can't do this on the fly; you have to have laid the groundwork for months," said Joshua T. Putnam, a political science lecturer at the University of Georgia. "By all accounts, the Trump campaign is not active in pushing their guys into those delegate slots."
By contrast, Senator Cruz, who has done well in caucus states, is seeking to get his supporters elected as delegates who are nominally pledged to Mr Trump, but who would desert him after the first ballot.
"The Cruz campaign has been organised down to the district and county levels all the way across the country," said Saul Anuzis, a former chairman of the Michigan Republican Party who has participated in meetings for the Cruz campaign about delegate selection. "You're dealing with people who are party activists. They will trump the Trump loyalists in winning delegate slots."
The Trump campaign seems to have recently awakened to this possibility. Friday, it announced a new "delegate selection team" of four people, led by Ed Brookover, a former campaign manager for Ben Carson with long experience as a Republican operative.
Mr Brookover said the campaign is working to get Trump supporters selected as delegates, including in Georgia, building on state operations in place during primaries and caucuses.
"The good news is, in going through my review of Mr. Trump's operations in these states, these folks have not dropped the ball," he said.
A handful of states and territories give candidates a direct say in naming delegates, but 44 states do not, and those account for about 73 per cent of all delegates, according to Benjamin Ginsberg, who was the national counsel to Mitt Romney's 2012 campaign.
"It's this under-the-radar competition and exercise of massive proportions that campaigns really need to be doing," Mr Ginsberg said. He recalled attending several state conventions four years ago where Romney supporters were "rolled" by delegates loyal to Representative Ron Paul of Texas, embarrassing Mr Romney with a protest at the national convention.
Ohio is one of the few states that allow candidates to submit their own list of delegates. Many others hold state conventions to elect them, and in some cases executive committees of the state party pick delegates, often from a list of donors or party stalwarts.
John Yob, a Republican strategist who was Rick Santorum's national convention director in 2012, coined a name for delegates pledged to one candidate but quietly sympathetic to another: "supporters in name only".
"There is almost 100 per cent certainty there will be people elected" as delegates in states across the country, he said, "that do not support the candidate they say" they back.
"The ultimate nominee will likely be determined in the convention because of the strength of the ground games at state conventions across the country," he added.
Another complication for Mr Trump is that party activists, who have historically dominated the state conventions where delegates are elected, tend to favour people with ties to the party establishment. Mr Trump's powerfully insurgent campaign has been anti-establishment since Day 1, and he has attracted few endorsements from Republican leaders. He faces the challenge of getting his outsider troops to attend arcane, lengthy and often boring meetings.
Mr Trump, who on Tuesday won decisively in Florida, the largest of five states that voted, still has the most likely path to a majority of delegates among the three remaining Republicans.
But as the race moves to the west and north-east, and with many states awarding most delegates based on votes by congressional district, Senator Cruz and Mr Kasich may be able to hold back Mr Trump in Indiana, Wisconsin, Washington, New York and California.
If Mr Trump continues to pick up delegates at the rate he has won them so far, he will finish with about 1148, which is 89 short of the majority needed to lock up the nomination.
Senator Cruz's path to a majority is harder: at his current pace, he would fall 559 short of a majority.
Mr Kasich, who has no mathematical path to a first-ballot victory in Cleveland, is preparing for a convention-floor fight. He announced late on Tuesday that he had hired a new adviser, Stu Spencer, who in 1976 helped President Gerald Ford secure the nomination over Ronald Reagan, the last time no candidate arrived at a Republican convention with a majority.
The most recent conventions that went into multiple ballots were even longer ago: when the Democrats met in 1952, and the Republicans in 1948.
Forty years ago, President Ford was able to wrangle a majority before the first ballot because many delegates came to the convention unbound. This year, because of rules changes, fewer than 10 per cent of Republican delegates will be free agents before the first ballot. They include some, but not all, of the 163 delegates Senator Marco Rubio won before quitting the race on Tuesday.
Though Mr Trump may hope to fish in this pool to close the gap and reach a majority before the first ballot, many of Senator Rubio's free agents could prefer Mr Kasich. Senator Rubio had told supporters in Ohio to vote strategically for the governor to deny Mr Trump victory.
There is a further pool of more than 100 unbound delegates, including part of the Pennsylvania delegation and from a handful of small states and territories. Mr Yob was elected as one of nine unbound delegate from the Virgin Islands on March 10.
"We've heard from all the campaigns," he said.