Nihilism . . . is a philosophical position which argues that the world, especially past and current human existence, is without objective meaning, purpose, comprehensible truth, or essential value. Nihilists generally assert some or all of the following: there is no reasonable proof of the existence of a higher ruler or creator, a "true morality" does not exist, and secular ethics are impossible; therefore, life has no truth, and no action can be preferable to any other.
Not every question has a right answer, but a great many questions in life, both factual and philosophical have wrong answers. Indeed, a belief that mere mortals are capable of discerning metaphysical truth is axiomatic to the atheism that is supposedly a predicate to nihilism. Metaphysics, of course, like physics, biology, chemistry and history viewed as a set of true statements about factual reality, differs a great deal from branches of philosophy and theology concerned with the meaning of law, its purpose, and morality.
Science can tell you how sharp a knife is, how strong it is, and whether it will tarnish in the dishwasher. It won't tell you what the best knife is, however, because that depends upon what the person using it needs. A knife too sharp can easily cut you. A knife too rigid is a poor choice to fillet a fish. You can cast that as moral issues which science can't answer. But, you can also cast that as an issue of context. Different answers make sense for different contexts -- but that doesn't mean that there isn't such a thing as a perfect fillet knife.
The notion that "true morality" does not exist or that secular ethics are impossible, while reasonable on the surface from the point of view of a secular worldview, appears less sensible when put into practice. While there are fine points where the common sense muddling through approach to secular humanist morality doesn't resolve well, developing a wide consensus on what is right and wrong, even on a vast international and multicultural scale, is far less of an ambitious thing than the nihilism suggests that it should be.
Often, the issues that are hardest to resolve as right or wrong are so hard to resolve because they are unimportant practically, and hence can't be subjected to empirical analysis. The death penalty, for example, in controversial, in part, because it impacts so few cases and has such a small impact on crime rates. No one disputes the people who commit heinous murders need to be removed from society, at least until they are too feeble to represent a continued threat to society.
At other times, divisions exist because the conventional wisdom of a large group of people is factually wrong in their beliefs. A majority of people on the eve of the Iraq War thought that Iraq played a part in 9-11. Most supporters of abstinence only education sincerely believe that contraception education causes teens to increase their sexual activity because there is official recognition of the legitimacy of teen sex. Few Creationists have a firm grasp on the science behind evolution. Most people who play the "Socialized medicine" card in the health care debate have little first hand experience of how well those systems actually work.
Jump ahead a moment. Suppose that, even if there is not necessarily one "true morality" that there is, at least, vast swath of moral territory which is immoral, through some combination of the hard wiring that comes with our species and the conclusions inevitably reached from living in the social communities that are for us, as social animals, our natural state.
It follows that the positivist view of law is wrong. There is some measure of natural law. To describe a law as a "bad law" or "unjust" isn't a inherently subjective rabbit hole.
Likewise, Democracy, in the absence of a nihilist worldview, means something very different than a political scientist's nihilist democracy, in which the majority view (or perhaps a utilitarian Pareto-optimal state) in ipso facto right. Voters, in a non-nihilist democratic model, are not principals. They are people who, like other actors in the political sphere, have a duty to act with reasonable care and a loyalty to the public good to secure a better regime, even despite their own narrow self-interests.
Political leadership, likewise takes a very different character in a non-nihilist worldview. The job of a political leader, is not just to carry out the will of the people, who are his or her masters, but to discern where the good society lies and persuade the people to go there. In this view, the politicians job is to get the people to buy into what is already right -- to educate the people.
A view that natural law exists in some form has explanatory virtue. For example, it makes it much easier to explain the voluntary democratic expansion of the franchise in the past two centuries. It isn't the only explanation, but it is a straight forward one.
Of course, if there is a secular natural law, one also needs a secular counterpart to original sin and the Tower of Babel. If there is a right and wrong in the world, why is it so hard to reach a consensus upon its terms?
But, sometimes, political division looms large only because we are focused on it. We lavish attention on flash points -- 5-4 decisions in the Supreme Court and party line votes in legislatures. But, a very large share of any court docket is decided unanimously, and the greater share of the legislative agenda in any session is dispatched with minimal dissent.
American political liberals and conservatives may seem hopelessly at odds, but there is widespread consensus on a great many issues. Democrats and Republicans may fight over how long sentences should be in absolute terms, but in the case of traditional violent and property crimes there is little dispute over what should and should not be criminal, and over the relatively seriousness of reasonably similar crimes.
Public policy empiricists are increasingly breaking political deadlocks. They are driving more public health oriented approaches to drug laws, a boldness in restricting without banning smoking, and increased regulation of teen drivers. They illustrate the failures of laws banning sex offenders from living in certain areas and disparate sentencing of crack and powder cocaine offenders. They are helping to make clear that our health care system is broken. They are cementing the notion that homosexuality is an inherent part of a person, rather than a "lifestyle choice", and that homosexual families can provide good homes for children. They assail worthless abstinence only education, and highlight the downsides of strict abortion bans.
Reality based public policy hasn't proven to be a terribly swift force for correcting political imbalance, but it has established a powerful new way to build consensus. Empirical analysis of public policy often informs the debate to make the moral disagreements trivial. Partisan ideas are increasingly being revealed as differences in empirical assumptions rather than differences in values.
Certainly, there are people who believe weird things and have moral systems at odds with those of almost everyone else. But, while they are disproportionately represented in politics, it takes rigorous indoctrination to bring someone to this state. A politics without dissent isn't possible or desirable. But, people of good will can, with good evidence, come closer together on most issues, because they often don't differ that much in their sense of right and wrong, if they perceive the same facts. The differences are often more over policy applications than in core principals, missions and values. Changing conventional wisdom about what works based upon empirical evidence is the challenge. Political leaders must evangelize discovered wisdom to secure political change, not just marshal majority coalitions of the status quo.