The comment below provides some resources to answer your question. As the question will at least for a time remain on our site, I personally believe it is important to stress that your question is based on a false premise - that there is in any way an equivalence between "those who say the Holocaust happened and those who say it didn't." The Holocaust is a matter of historical record. Holocaust denial is a fringe belief that is widely discredited. Traditions of freedom of expression have allowed those who deny the historical record to have a voice, but that does not mean that activists who believe in social justice have to pretend that such views have any credence.

The struggles of some western governments to deal with holocaust denial within a free speech framework is the subject of this article:
The Memory of Judgment: The Law, the Holocaust, and Denial
Lawrence Douglas
History and Memory
Vol. 7, No. 2 (Fall - Winter, 1995), pp. 100-120
available via JSTOR