While Donald Trump is not yet the official Republican Nominee, it is highly likely he will be. There isn't a competitor left running against him.
Is there a legal mechanism that can be employed to block his assumed nomination?
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There are very few, if any, federal or state legal constraints on what a political party may do in selecting its nominee. The US Constitution simply does not allow federal and state laws that limit internal party decisions.
As for party rules being a constraint, the party delegates make the nominee decision by voting on who will be the the party's nominee. A simple majority wins: the first candidate to get a majority of the delegates' votes wins. The delegates operate within rules that are subject to change by the delegates themselves, and they will make their decisions on rules and voting based on many highly variable considerations, including political influences, self-interests, party interests, and their own moral considerations, if they have any.
In the final analysis, the delegates will be able to do pretty much whatever they want to do without any significant restraints imposed by federal or state law. However, if Mr. Trump has already the commitment of a majority of the delegates before the convention, his nomination is virtually assured. If Mr. Trump has the commitment of 49%, then the delegates will very likely take into account political concerns arising from very bad appearance if they deny Mr. Trump a nomination that many would feel he "deserved," but legal considerations or party rules would not be a major constraint on delegates' decision to vote for or against Trump after the first ballot.