"This constant lying is not aimed at making the people believe a lie, but at ensuring that no one believes anything anymore."

Hannah Arendt (14 October 1906 – 4 December 1975) 

German historian and philosophe 

"This constant lying is not aimed at making the people believe a lie, but at ensuring that no one believes anything anymore.
    "A people that can no longer distinguish between truth and lies cannot distinguish between right and wrong.
    "And such a people, deprived of the power to think and judge, is, without knowing and willing it, completely subjected to the rule of lies. With such a people, you can do whatever you want."

~ attributed (incorrectly) to Hannah Arendt

NB: The Hannah Arendt Centre says "the spirit of the quote is very much in line with Arendt’s own thought. But as far as I can tell, Hannah Arendt never said this or wrote this. She did, however, say many similar things. ..."

The closest in spirit and content, and also the most easily available, is from an interview with Roger Errera in 1974, what turned out to be Hannah Arendt’s last public interview. Arendt spoke about the importance of a free press in an era of mass manipulation of truth and public lying: She said:

"The moment we no longer have a free press, anything can happen. What makes it possible for a totalitarian or any other dictatorship to rule is that people are not informed; how can you have an opinion if you are not informed? If everybody always lies to you, the consequence is not that you believe the lies, but rather that nobody believes anything any longer. This is because lies, by their very nature, have to be changed, and a lying government has constantly to rewrite its own history. On the receiving end you get not only one lie—a lie which you could go on for the rest of your days—but you get a great number of lies, depending on how the political wind blows. And a people that no longer can believe anything cannot make up its mind. It is deprived not only of its capacity to act but also of its capacity to think and to judge. And with such a people you can then do what you please."

The key point in Arendt’s statement is that as lies multiply, the result is not that the lie is believed but that people lose faith in the truth and are increasingly susceptible to believe anything. When cynicism about truth reigns, lies operate not because they replace reality but because they make reality wobble–a phrase Arendt employs in her essay Truth and Politics. In that essay, Arendt argued that mass lying undermines our sense of reality by which we find our bearings in the real world: 

The result of a consistent and total substitution of lies for factual truth is not that the lie will now be accepted as truth and truth be defamed as a lie, but that the sense by which we take our bearings in the real world—and the category of truth versus falsehood is among the mental means to this end—is being destroyed.

The Arendtian point is that constant lying by a propaganda machine does not lead to the lie being believed but leads, instead, to cynicism.

Which is all that the liars need.