[F]actual truth is no more self-evident than opinion, and this may be among the reasons that opinion-holders find it relatively easy to discredit factual truth as just another opinion. Factual evidence, moreover, is established through testimony by eyewitnesses – notoriously unreliable – and by records, documents, and monuments, all of which can be suspected as forgeries. In the event of a dispute, only other witnesses but no third and higher instance can be invoked, and settlement is usually arrived at by way of a majority; that is, in the same way as the settlement of opinion disputes – a wholly unsatisfactory procedure, since there is nothing to prevent a majority of witnesses from being false witnesses. --Hannah Arendt
In totalitarian regimes, which twisted facts to suit their “truth”, majorities of witnesses could be “encouraged” to bear false witness. In democracies majorities can be convinced that an opinion is a factual truth. In both cases, though a citizen armed with facts could hold power to account. Yet, for the citizen to hold power to account facts have to be embedded in the truth or political knowledge, which is why totalitarian regimes rewrite their history books. The totalitarian rulers know that the nature of political things cannot be changed or made otherwise by fiat, so they do the next best thing, they change the opinion about those facts.
In a democracy, political things are confused with everyday things. Even though buying a shirt would never be confused with casting a vote, they are often equated as a similar choice. What differentiates them though is that political things exist within a context that shows their political nature. To cast a vote, a citizen requires a ballot and an election. The ballot, the ballot box, and the election are political facts that create the context for political things to be revealed. Decent politics exists where people replace opinions of political things with knowledge of political things. The knowledge comes from the study of political things that are anchored in reality by facts. A political fact torn from such context is easily spotted as a falsehood such as claiming Abraham Lincoln died in 1965 or Donald Trump won the 2016 Iowa Republican Caucus. However, political knowledge is often confused with political opinions that reflect prejudices or guesses about political things rather than knowledge of the political things. Unlike a totalitarian regime which often relies on violence to enforce its rule, the democracy becomes distorted as political opinions begin to replace facts.
In a democracy, those who want to control the public domain, who resist the effort to obtain political knowledge, will seek to reduce facts to opinions. The goal is to reach a state where any “fact” can be shaped to fit the “truth” the speaker wants to claim. A prominent example of this trend is Donald Trump's doubts about Barack Obama's birth certificate. He wants his opinion, that the President’s birth certificate is fraudulent, to replace the fact. He asserts his opinion to create a doubt about the facts so that they can be dismissed. Were he to achieve this, he could have greater power in the public domain for it would reflect his opinion and no the facts. However, to do this he has to pull apart the context within which the political fact exists, which is what makes the birth certificate valid. He has to dismiss the birth registration process, the bureaucratic context, and the integrity of all the people involved.
Democracies have always been susceptible to demagogues. They want their opinion to be the basis by which the community acts and to do this they have to discredit political knowledge and political facts. When “truthers” seek to discredit political facts as a common standard for behaviour within the public domain their opinions take root. Such opinions flourish when they begin to replace the common standard of truth. Without a common standard of truth, based on political facts, the common good that binds the country begins to fray. To destroy this common good, a liar that is one who wants to replace facts with opinions, will shape his “facts” to fit what his audience expects. Through this method, he undermines those who have political knowledge or political facts, a truth teller as unpersuasive.
If it be true that all governments rest on opinion, it is no less true that the strength of opinion in each individual, and its practical influence on his conduct, depend much on the number which he supposes to have entertained the same opinion. The reason of man, like man himself, is timid and cautious when left alone, and acquires firmness and confidence in proportion to the number with which it is associated. --James Madison Federalist #49
With truth tellers discredited, the “truthers” can impose their opinions as the truth without concern for verifiable political facts. Moreover, the “truther” will insist that any facts they disagree with are simply opinions. (It is your opinion that Obama’s birth certificate is valid. It is your opinion that steel will soften and melt under intense heat of burning jet fuel). The political is defended as a constitutional right. The “truther” wants to claim that it even if his political opinion is not true or factual he is entitled to speak it as constitutional right. He will assert that he has a right to be wrong. The danger is that no one has a right to be wrong about facts; Germany did invade Belgium, the Declaration of Independence was signed in 1776, Trump did lose in Iowa. Without the insistence on facts and the truth derived from them, decent politics begins to wither. The belief that that opinions replace facts means that a shared understanding of political things begins to fray. When opinions replace facts as the basis for political truth, when the difference between right and wrong is reduced to an opinion, then freedom based on the common good becomes doubtful.
 TRUTH AND POLITICS by Hannah Arendt Originally published in The New Yorker, February 25, 1967, and reprinted with minor changes in Between Past and Future (1968) and The Portable Hannah Arendt edited by Peter Baier (2000) and Truth: Engagements Across Philosophical Traditions edited by Medina and Wood (2005) p. 304