Washington: Republican delegates -- designated during the presidential primary process -- will choose the party's candidate for the November election at a nominating convention in Cleveland in July.
But frontrunner Donald Trump's rivals are hoping to prevent him from achieving the majority of delegates needed to seize the prize -- a situation that would result in a brokered convention.
According to Republican party rules, each US state and a handful of territories send a certain number of delegates to the convention to elect the White House nominee. The rules for choosing those delegates vary from state to state.
In Florida, for instance all 99 delegates went to Trump as the winner of Tuesday's primary, while in New Hampshire, delegates were awarded on a proportional basis.
Over the course of the primary campaign, each candidate amasses delegates. For the past four decades, the frontrunner has always reached the magic number needed to win the nomination.
This year, that number is 1,237 -- the majority of the 2,472 delegates in play.
But the strength of resistance to Trump's candidacy, still opposed by rivals Texas Senator Ted Cruz and Ohio Governor John Kasich as well as the bulk of the Republic establishment -- makes it possible that he may fall short.
If that were to happen, the outcome would be what is known as a brokered convention in which the delegates -- who normally play a purely symbolic role, effectively rubber-stamping the results of the primaries -- acquire a critical influence over the nomination.
For the first round ballot, party rules oblige delegates to back the candidate to whom they were pledged in the primaries. Those tied to candidates no longer in the race, such as Marco Rubio and Jeb Bush, would not vote.
But based on the results of the primaries, that first round would not produce a majority, and the vote would go to a second round.
"The majority of states free their delegates after the first ballot," explains Josh Putnam, a lecturer in the department of political science at the University of Georgia and a campaign expert.
That means those delegates could change their votes -- and may gravitate towards an alternative candidate.
- Rare occurrence
Such a situation is possible because in a bit less than 75 percent of US states, the selection of convention delegates is done without input from the candidates themselves, Ben Ginsberg, a former Republican National Committee lawyer, told MSNBC.
The delegates are chosen in local party conventions over the coming months. So, a Trump delegate could choose to vote for Kasich or Cruz in an eventual second round.
According to The New York Times, 57 percent of the delegates would be freed up to change their votes in the second round, and 81 percent would be freed up in a third round. There is no limit on the number of rounds before a candidate earns a majority.
There is one caveat: a committee will meet a few days before the convention at which point they could decide to change the convention rules.
Brokered conventions are rare in modern American politics, with the parties invariably finding complex maneuvers to avoid such a scenario.
In 1976, no Republican candidate had a majority. Incumbent president Gerald Ford was facing Ronald Reagan. After several days of deal-making, Ford won in the first round thanks to support from uncommitted delegates.
The last time that several vote rounds were needed came in 1948, when Thomas Dewey was the eventual candidate. For the Democrats, the last brokered convention was in 1952, with three rounds necessary to pick Adlai Stevenson.