Rule 9 of the Republican Party rules covers 'filling vacancies in nominations."
It says that the Republican National Committee is "authorized and empowered to fill and all vacancies, which may occur by reason of death, declination, or otherwise" of the candidate.
Under the rules, the Republican National Committee has two options. It could reconvene the entire convention – all 2437 delegates. Or, a much more likely options, would be to convene the Republican National Committee's 168 members, consisting of 3 members from each state and territory, to represent the delegates.
Each member would represent one-third of the number of delegates awarded in each state and cast a vote for a presidential candidate.
For instance, three RNC members from California would represent the state's 99 delegates. If two candidates cast a vote for made-up candidate Jan Smith and one member casts a vote for the other made-up candidate Max Brown, then Smith would receive two-thirds of the delegates – or 66 delegates – and Brown would receive one-third – or 33 delegates.
The candidate that wins 1,237 delegates would be named the new nominee.
