Modern liberal democratic states are oligarchies of state-capitalist power elites in practice (C. Wright Mills). But they have to afford a reasonable standard of living, level of protection, and quality of life to the middle class in order to maintain their legitimacy. States tend to collapse when they can no longer hold the support of the middle class. The middle class generally fears the lower class more than the ruling class (for a range of reasons, e.g. crime, economic competition, perceived cultural threats, status anxiety, etc). So the state will maintain the loyalty of the middle class by ensuring the lower class is effectively suppressed. Political rivalries in liberal democracies either represent different factions of the elite attempting to build constituencies for themselves (e.g. FOX News or MSNBC) or various middle class factions seeking favors and preferential treatment from the state and other elite institutions.
For instance, most “conservatives” complain endlessly about “big government” but what they mean is “Cut my taxes and balance the budget, but don’t eliminate any social insurance programs I personally benefit from or take away my government job, and by all means arrest those crack smoking colored folks, and military spending can never be high enough!.” Most “liberal” constituencies are comparable in the sense of wanting a range of institutional assistance in terms of advancing their own interests within the state-capitalist system (e.g. “more black/brown/female faces in high places,’ subsidizing high-tech industries, protecting the public sector employment of middle class bureaucrats, keeping guns out of the hands of those nasty rednecks and ghetto folks, etc.)
It’s true that class divisions are widening in the US, and are now the widest they have been in about 100 years, although that has to be considered in light of the fact that most poverty in the US is relative poverty rather than absolute poverty. However, the ranks of the absolutely poor are growing as well. The red/blue divide reflects an intra-middle-class rivalry, but it’s also true that class divisions are starting to show within both tribes. I interpret the support for Trump and Sanders in the primaries last year to have been largely representative of insurgencies by the working class and sinking middle class within both parties. But none of that represents “the poor” per se, just the sectors of the middle class and upper strata working class/lower middle class that are in a state of decline.
Liberal democratic states begin to lose their legitimacy when the ranks of the poor grow large enough to rival the middle class as a socioeconomic demographic. The middle class will typically begin to fracture in such a scenario. Some middle class sectors will strengthen their loyalty to the ruling class, others may join the ranks of the lower class (whether politically, economically, or both). The middle class may also fragment into rival factions that oppose both the ruling class and the lower class (for example, both communist and fascist movements tend to be heavily dominated by those who are drawn from the ranks of the economically and/or politically frustrated sectors of the middle class or upper strata working class/lower middle class).
The loss of legitimacy by a liberal democratic state usually results in one of two scenarios. One is that the power elite abandon all pretenses of “democracy,” “constitutional rights,” or popular participation in government, and rule by sheer state repression and state terrorism. Classic examples would be the death squad regimes that dominated much of Latin America in the mid to late twentieth century. The other scenario is one where dictators and demagogues come to power in part by promising to help the poor (which is a major aspect of how fascist and communist dictators typically achieve power). However, a liberal democratic state will typically not lose its legitimacy until the ranks of the absolute poor grows to a highly significant level.
While the ranks of the absolute poor are growing in the United States, and the US is likewise moving towards the development of a traditional two-tiered Latin American-like class system, the present class divisions are representative of the emergence of a super rich class of billionaire plutocrats and an affluent upper middle class of the newly rich from outside the tradition elite on one hand, and the growth of the ranks of the relatively poor due to a range of economic, political, demographic, and technological factors. This inturn means that the likely politico-economic future of the US will more closely resemble the contemporary Brazilian model than that of the death squad states or left/right dictatorships of the twentieth century.