Proof by Assertion is a technique used in advertising, and is exemplified in a quote by Vladimir Lenin: "A Lie told often enough becomes the truth."
Other versions:
"If you repeat a lie often enough, it becomes the truth."
"If you tell a lie big enough and keep repeating it, people will eventually come to believe it."
"If you repeat a lie often enough, people will believe it."
"If you repeat a lie long enough, it becomes truth."
"If you repeat a lie many times, people are bound to start believing it."
"Say something loud enough, often enough, and forcefully enough, and those listening will eventually accept it as the truth."
These have been attributed to many sources, including Joseph Goebbels. Speaking of Nazis (Godwin!)...
"
But the most brilliant propagandist technique will yield no success unless one fundamental principle is borne in mind constantly and with unflagging attention. It must confine itself to a few points and repeat them over and over. Here, as so often in this world, persistence is the first and most important requirement for success." -- Adolph Hitler, from "War Propaganda", in volume 1, chapter 6 of Mein Kampf (1925)