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.
You describe the danger from the lower classes. Upper class can also abuse the system by buying/financing candidates and buying/lobbying elected or non elected officials.
The most productive class is the middle class, presumably.
Republic means - Res Publica - Common Good or Commonwealth. It dies when the groups or individuals care only for their own.