(Rather than try to say exactly what Hitler or Goebbels meant by the concept as an historical thing, this is my general conception of it.)
From the United States Office of Strategic Services psychological profile of Hitler:
His primary rules were: never allow the public to cool off; never admit a fault or wrong; never concede that there may be some good in your enemy; never leave room for alternatives; never accept blame; concentrate on one enemy at a time and blame him for everything that goes wrong; people will believe a big lie sooner than a little one; and if you repeat it frequently enough people will sooner or later believe it.
The BIG LIE as a propaganda method is based on the idea that enormous lies are believed more readily than lesser lies, and that if you never waver from the big lie people will end up accepting it as at least partially true.
The really BIG lie is almost a religious experience. It is so divorced from reality as to be transcendental and will involve parts of the brain that do not reason. A big enough lie leap-frogs our reasoning capacity and goes right into, "does it FEEL correct?"
It is human nature to believe everything we hear. Belief is our default position. This makes sense when you consider how best to respond to, "Don't drink that, it's poison," or, "there's a bear in that cave."
When someone rolls out the big lie we cannot accept that a person would tell SUCH a big lie, and will eventually frame it as an exaggeration. "The Jews are responsible for all of Germany's defeats" becomes, "The Jews are one of many factors responsible for Germany's defeats." If you get people to 'split the difference' between the truth and a gigantic lie, the lie is halfway home. That's the idea behind teaching both evolution (true) and intelligent design (big lie). Splitting the difference is a big step forward for the lie.
And repetition simulates consensus. One person saying something a thousand times is like a thousand people saying it once. If you keep hearing it the cognitive dissonance and desire to believe wear down your mental resistance. "Everybody is saying it so there must be
something to it."
And this is why Republicans intentionally lie, and stick with their story.
The press will bitch for a while about how Palin's entire stump speech is false, but they will be worn down over time. Soon it will be considered "a little resume padding" rather than a series of flat lies.