Bulverism is a form of fallacious argument in which one distracts from the task of looking for a correct answer by means of explaining why one's opponent is wrong.
"If I can give a reason why my opponent is wrong I do not have to explain how he is wrong."
C. S. Lewis described this type of fallacy in 1941, eventually publishing an essay on it in The Socratic Digest under the title Bulverism:
"You must show that a man is wrong before you start explaining why he is wrong. The modern method is to assume without discussion that he is wrong and then distract his attention from this (the only real issue) by busily explaining how he became so silly.
In the course of the last fifteen years I have found this vice so common that I have had to invent a name for it. I call it 'Bulverism'. Some day I am going to write the biography of its imaginary inventor, Ezekiel Bulver, whose destiny was determined at the age of five when he heard his mother say to his father -- who had been maintaining that two sides of a triangle were together greater than a third -- 'Oh you say that because you are a man.' 'At that moment', E. Bulver assures us, 'there flashed across my opening mind the great truth that refutation is no necessary part of argument. Assume that your opponent is wrong, and explain his error, and the world will be at your feet. Attempt to prove that he is wrong or (worse still) try to find out whether he is wrong or right, and the national dynamism of our age will thrust you to the wall.' That is how Bulver became one of the makers of the Twentieth Century."
This type of error can be achieved through the use of many different fallacies, but in practice the majority of examples are ad hominem circumstantial -- "you believe X because you are different from us in way Y". This is essentially the form of argument most often used in political debate: "my opponent is directed by special interest groups"; "he is blinded by tradition"; "Republicrats have to say that in order to be elected". There is good reason for this -- in a debate, time spent correcting your opponents' arguments takes up your valuable speaking time, has the potential side effect of making their positions stronger, and if you discover that their argument is sound, you might have to change your opinion.
Learning to recognize this sort of evasion quickly -- quickly enough to catch it in conversation -- takes practice. Moreover, it is one of the harder fallacies to correct, because the person making it has essentially declared that they have neither the time nor the interest for this subject. To correct this error, seize the conversation with a firm hand, and demand to know either what the error in reasoning is or, alternatively, what a person not making your fatal error would do to correct it. You may, at times, have to do this repeatedly. You should continue this until the other person leaves, probably screaming obscenities at you.