The republics die when their elites become corrupt, decadent and greedy and the lower classes become a mob.
Thus Spake [Nietzsche's] Zarathustra:
[...] Fretted conceit and suppressed envy--perhaps your fathers' conceit and envy: in you break they forth as flame and frenzy of vengeance.
What the father hath hid cometh out in the son; and oft have I found in the son the father's revealed secret.
Inspired ones they resemble: but it is not the heart that inspireth them-- but vengeance. And when they become subtle and cold, it is not spirit, but envy, that maketh them so.
Their jealousy leadeth them also into thinkers' paths; and this is the sign of their jealousy--they always go too far: so that their fatigue hath at last to go to sleep on the snow.
In all their lamentations soundeth vengeance, in all their eulogies is maleficence; and being judge seemeth to them bliss.
But thus do I counsel you, my friends: distrust all in whom the impulse to punish is powerful!
They are people of bad race and lineage; out of their countenances peer the hangman and the sleuth-hound.
Distrust all those who talk much of their justice! Verily, in their souls not only honey is lacking.
And when they call themselves "the good and just," forget not, that for them to be Pharisees, nothing is lacking but--power![...]
True enough. The other primary killer of a republic is when the people learn of the ability they have to vote themselves moneys out of the public purse. There is a huge class of people that will vote to enslave others to provide for their leisure. Democrats pander to this mentality to garner the votes of the non-productive class. This pandering also neatly dovetails with their use of rivalry and prejudice to set one group against another and thus keep them in bondage and on the plantation.
This could be dealt with, I think, in the long run, if the Republicans did not similarly lust for the power and money inherrant in what the system that our republic has degenerated to. I believe that the symbol that should be adopted by both major parties would be a portrait of Janus.