From a legal standpoint, is it possible for a candidate to bypass the primaries/caucuses and be nominated as an official party candidate? For instance, could Mitt Romney, in spite of the fact that (up until now) he did not take part in the primaries, become the Republican nominee?
Important note: This is merely and strictly a factual question about the political system in the US, not about the personalities and/or political agendas of Donald Trump, Ted Cruz, Marco Rubio and Mitt Romney.