The parties modify their own rules on a regular basis, with Colorado recently becoming infamous for selecting delegates based on an incredibly rushed process, and without any popular vote to weigh against.
However, the Constitution made no provision for political parties. The founders did not envision a United States divided between and under the control of political parties. In fact, George Washington warned of the dangers of political parties in his farewell speech.
The alternate domination of one faction over another, sharpened by the spirit of revenge, natural to party dissension, which in different ages and countries has perpetrated the most horrid enormities, is itself a frightful despotism. But this leads at length to a more formal and permanent despotism. The disorders and miseries, which result, gradually incline the minds of men to seek security and repose in the absolute power of an individual; and sooner or later the chief of some prevailing faction, more able or more fortunate than his competitors, turns this disposition to the purposes of his own elevation, on the ruins of Public Liberty
Without looking forward to an extremity of this kind, (which nevertheless ought not to be entirely out of sight,) the common and continual mischiefs of the spirit of party are sufficient to make it the interest and duty of a wise people to discourage and restrain it.
It serves always to distract the Public Councils, and enfeeble the Public Administration. It agitates the Community with ill-founded jealousies and false alarms; kindles the animosity of one part against another, foments occasionally riot and insurrection. It opens the door to foreign influence and corruption, which find a facilitated access to the government itself through the channels of party passions. Thus the policy and the will of one country are subjected to the policy and will of another.
There is an opinion, that parties in free countries are useful checks upon the administration of the Government, and serve to keep alive the spirit of Liberty. This within certain limits is probably true; and in Governments of a Monarchical cast, Patriotism may look with indulgence, if not with favor, upon the spirit of party. But in those of the popular character, in Governments purely elective, it is a spirit not to be encouraged. From their natural tendency, it is certain there will always be enough of that spirit for every salutary purpose. And, there being constant danger of excess, the effort ought to be, by force of public opinion, to mitigate and assuage it. A fire not to be quenched, it demands a uniform vigilance to prevent its bursting into a flame, lest, instead of warming, it should consume.
I do not see how he could be wrong. Parties, like any other group, can be corrupted and co-opted by foreign or business interests that have goals contrary to the rights of the people. The establishments of both parties are fighting hard to maintain the status quo. Bernie Sanders's policies are certainly unworkable on their face, and it is not known if Donald Trump can (or would) keep his promises, but maintaining the status quo is not a solution to the nation's problems either.
The party system should be abolished, and there should be a single standard for the delegation process. Candidates should stand or fall on their own merits, and not on the whims of a party's (and donor's) interests.
The likelyhood of such a reformation is virtually nil, because the powers that be would not benefit. Many Americans would not accept it either, because there are people on both the left and right who see parties - and by extension government - as a vector to enforce THEIR morals on others.