Candidates who drop out release their delegates usually asking them to vote for another candidate.
At convention time the first roll call is done. The vast majority of delegates are pledged\bound to a candidate. If no one has 50% + 1 of the votes there is a short recess and then a revote. The difference being that delgates are no longer pledged or bound to a particular candidate.
At this point ANYONE can be vote for.
The last time this happened by '76 Ford V. Reagan.
In 1924 it went 103 rounds.
And Reagan’s challenge so weakened incumbent President Ford that we ended up with four miserable years of Jimmy Carter.
Unintended consequences, I know, but it is important to remind ourselves that no man is perfect, even the now-sainted Reagan.
There are problems with your examples in using them to equate that a totally "fresh" candidate can be injected now. In the Reagan Ford battle, both were actual candidates in the Primaries. There was no injection of another candidate. In 1924, we didn't have a Primary system like today. The state GOP Grandees and Poobaa's of the parties decided who they wanted to nominate behind closed doors without any voters deciding. There is no way that will ever happen again.
A brokered convention is not going to work out anything like what people here are romanticizing about. One of the 4 candidates in the race today will be our nominee, guaranteed. The only thing a BC will do is allow the Party hacks to weight it towards their guy Romney.
Section 10. Pledge of Delegates and Alternates.I expect two of the candidates will team up to join delegates before enough delegates are released to bring another nominee to the floor, if that's even a possibility in National GOP rules. Even if the lesser candidate does not get all of his delegates to go along, it may be enough without going past the first couple of rounds of voting.a. Commitment to Candidate: Following canvass of the presidential primary by the SREC, the State Chairman shall, in a manner directly proportional to the statewide presidential vote, as well as the presidential vote by congressional district, if possible, assign each delegate to represent a Presidential candidate (or uncommitted), and upon that assignment each delegate and alternate representing a Presidential candidate becomes pledged to the Presidential candidate on whose slate the delegate and alternate is nominated in accordance with subsection b of this section. The State Chairman, National Committeeman, and National Committeewoman, who are national delegates under Rule of The Republican Party 13(a)(2) by the virtue of their office shall not become pledged to a candidate and remain uncommitted as specified below, nor shall they be counted as uncommitted for the purposes of proportional assignment made in this section.
b. Length of Commitment: A person who is elected as a delegate or alternate to the National Convention and who is assigned to a Presidential candidate by the State Chairman to represent that particular Presidential candidate at the National Convention and who does not resign from the position is pledged to support that Presidential candidate at the National Convention until the candidate is nominated or until the delegate or alternate is released from the pledges as follows:
1. First (1st) nomination convention ballot: delegate or alternate shall be released from the pledge only in the event of death, withdrawal, or by decision of the candidate. For the first ballot taken at the National Convention to determine the nominee of the Republican Party for the office of President of the United States, the totals of the votes of the members of the Texas delegation shall be announced as they would have been if the individual delegates had been awarded to or designated for the respective candidates for such office on the Texas General Primary ballot in accordance with the state-wide result of the of the voting for such candidates. No poll of the members of the delegation shall be taken for the announcement of the vote.
2. Second (2nd) nomination convention ballot: delegate or alternate may be released from the pledge by decision of the candidate;
3. Third (3rd) nominating convention ballot: delegate or alternate shall be released from the pledge if the candidate has failed to receive twenty percent (20%) or more of the total vote cast on the preceding ballot; or by the decision of the candidate;
4. Fourth (4th) and subsequent nominating convention ballots: delegates and alternates are released from any pledge.
c. Uncommitted Delegates: Uncommitted delegates and alternates may vote as they choose on all questions and candidates presented at the National Convention.