If history is a guide, Republics usually fail when an oligarchy seizes control. Oligarchies are not stable though, for a few reasons. One is that the oligarchs start competing with each other. Linked to that is that they disregard the health of state in their competition. The populace also becomes tired of paying the bills for oligarchs. Popular dissatisfaction, combined with oligarchical competition often leads to civil war and ruin.
The American oligarchy is now led by Charles Koch, who controls nearly all of the Republican party apparatus - nearly all of the congressional Republicans suck at the Koch teat and swear fealty to his program. A competitive oligarch, Donald Trump is challenging his rule and seems certain to get the Republican nomination. Of course his actual policy program, to the faint extent that he has one, is little different from the Kochs, but he is a challenge to their power.
The games begin.