"PHILOSOPHICALLY SPEAKING...":

Moral/political philosopher John Rawls is best known for his work on "justice" in the 1970s -- work I introduce in my "Philosophically Speaking" class and occasionally in my "Mission Possible: Global Issues, Leadership Choices" class -- but Rawls spent the last 20 years of his career considering the problems inherent in a democracy. This article from Philosophy Now (UK) provides a quick summary of Rawls's insightful (and prescient) 1993 work, Political Liberalism, which focuses on the democratically necessary ingredient of "reasonableness":  

"Political Liberalism starts with the observation that the cultural environment of modern democracies contains diverse religious and philosophically-based moral doctrines. Rawls argues that this diversity is not surprising, since the protection of personal freedom that democratic societies promote naturally leads over time to increasing diversity in what he calls ‘the background culture’, that is, civil society – the space where we cultivate our personal ideals and goals. Highlighting that the background culture of modern democracies is marked by diversity is nothing new or particularly illuminating. Most of us only have to look around to see that. What is new, which Rawls appreciated with incredible insight, is that this mounting diversity introduces a particular justificatory problem for democracy. The problem can be stated as follows: if the beliefs that we hold are conflicting and irreconcilable, which of them can be used to justify the democratic system itself? Put differently, if I am not willing to endorse a political system based on your beliefs, and you are not willing to accept one based on mine, then how are we going to set any common rules to help us live together? ...

"As Rawls defines the term, an idea or person counts as politically reasonable if they exhibit two main characteristics: 1) They have to respect the principle of democratic justification – meaning that they have to propose terms of social cooperation that others might also endorse; and 2) They have to recognize what Rawls calls ‘the burdens of judgment’ – the fact that other citizens can arrive at different beliefs in their honest search for truth. Putting these two elements together, we can say that a politically reasonable person is one who offers reciprocal terms of cooperation, refraining from using political power to favor their own worldview or repress the views of other reasonable people. ...

"If you and I and others fundamentally disagree in our beliefs, but if we all uphold the requirements of reasonableness – if we respect the need for democratic justification and accept that people will forever have different fundamental beliefs – then there will be enough overlap amongst us to facilitate consensus at a political level. The fact that our thinking is reasonable is what ties us all together despite our fundamental disagreements. Exactly how each religious or secular tradition justifies to itself the need to respect the virtue of reasonableness is not political liberalism’s concern. What matters is that they do so somehow. For a democracy to be stable in the midst of irreconcilable diversity, the doctrines of its citizens need to be reasonable. ...

"Unreasonable citizens reject the idea that their obligations as citizens take precedence over their beliefs. ... Rawls recognized that if citizens fail to uphold reasonableness, the whole edifice of liberal democracy would be in peril. ...  There is nothing political liberalism can say to those looking to elevate their own version of morality into the political sphere, other than flagging up the fact that they’re being politically unreasonable. ... Since we have little to say to unreasonable actors other than pointing out their lack of reasonableness – which is unlikely to make them lose much sleep – then the only thing left for political liberalism is 'the practical task of containing them – like war and disease – so that they do not overturn political justice'. By drawing the limits of what can be tolerated as reasonableness, political liberalism licenses the state to eradicate uncooperative fundamentalism within it like a disease." 

But, as with all diseases, success in curing the body politic depends on catching the problem in time -- and agreeing to the cure.
philosophynow.org/issues/122/Democracy_and_the_Unreasonable_Lessons_from_Rawls