For example, can Marco Rubio and Kasich give delegates to Ted Cruz in order to defeat Donald Trump?
Delegates are not controlled by their candidates. Candidates do have some influence though. They can
- Drop out. This usually releases all their delegates.
- Recommend someone. Candidate choices are not binding on their delegates, but they can have influence.
As a general rule, unless the candidate drops out, delegates are bound through the first ballot. They have to vote for the candidate to whom they were bound. They may still be bound on subsequent ballots. The rules vary by state.
If no candidate has a first ballot majority, almost anything can happen at the convention. The delegates themselves set the rules.