Questions about nomination process

As we watch the repub rats desert the sinking ship, exactly how responsible are they for the nominee?

When the 19 or so original candidates put their names forward, exactly what is the criteria for one of them to represent their party? I ask this as an interested 'foreigner' because even though we have some real bastards try to run for office, the party pre-selection process generally weeds them out so only the best, as it were, of the worst ends up with their names on the ballot.

Do the party machines go through any sort of vetting process before the start of the primaries or is it enough for a candidate to say they are a Republican and away they go? If so, do they not have the brains and the foresight to deal with the possibility of a totally unsuitable, unelectable candidate and short-circuit the pending disaster?

IMO the GOP needs to be wearing this debacle around their necks like a stinking fish for decades to come.