As a Usenet discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one.” There is a tradition in many groups that, once this occurs, that thread is over, and whoever mentioned the Nazis has automatically lost whatever argument was in progress.

From MSNBC:

In a speech to Israel's Knesset, Bush said: "Some seem to believe that we should negotiate with the terrorists and radicals, as if some ingenious argument will persuade them they have been wrong all along.  We have heard this foolish delusion before. As Nazi tanks crossed into Poland in 1939, an American senator declared: 'Lord, if I could only have talked to Hitler, all this might have been avoided.' We have an obligation to call this what it is — the false comfort of appeasement, which has been repeatedly discredited by history."

Sure sign of a weak argument when one party starts comparing the other to Nazis.