First, let's remember what slavery was - it was the condemnation of all Africans (American born and otherwise) to a life of servitude. And all it took was one drop of African blood to become a slave. So, if a person whose parents, grandparents, great-grandparents, and fifteen of their great-great grandparents were all European, and the sixteenth great-great grandparent was African, that person would be a slave unless proven otherwise. It was institutionalized discrimination. Moreover, once someone was a slave, they had no legal recourse to protect themselves. Slaves were regularly subject to torture, rape, and even murder and they had no recourse. Remember Solomon Northrup was born a free man, was kidnapped, tortured so horribly he was afraid to say his name, put to work without pay, beaten and tortured on a regular basis. He received not a penny of compensation for 12 years of servitude, beatings, and torture. Zip, zilch, nada. And he, as a free black man, had legal rights that slaves did not. Oh, and white men who knew he was a free black man were terrified of standing up for his rights because doing so would possibly subject them to extra-legal death and/or dismemberment.
As far as Nazism goes, let's remember that the Nazis were a basic totalitarian regime (i.e. North Korea), but much, much worse. For a blond-haired, blue eyed German who could trace his lineage back to Sigfried, life under the Nazi regime meant that he had to do everything through the Nazi Party. Speaking against the Party could (and generally did) mean arrest, torture, and maybe summary execution. For a Jew (or someone with a Jewish great, great grandparent), a Romani, or gay, life was significantly more difficult. At best, these individuals would provide slave labor, and unlike slavery in America, the Nazis had no economic incentive to keep Jews, Romani, or gays alive. In fact, the Nazis held middle-management meetings to discuss the most efficient way to slaughter millions solely on the basis of their ethnicity.
So, to help with analogies, let's keep in mind the following:
If the governmental policy you are protesting does not result you getting tortured, dismembered, raped, murdered, or forced to perform manual labor for no compensation because of your ancestry, IT IS NOT LIKE SLAVERY.
If the governmental policy you are protesting does not turn you, or your loved ones into property, and does not allow for someone else to sell your children to the highest bidder, IT IS NOT LIKE SLAVERY.
If the governmental policy you are protesting does not result in the systematic and summary execution of millions of people by the most efficient means possible, IT IS NOT LIKE NAZI GERMANY.
If the governmental policy you are protesting does not result in your total and complete loss of any and all due process rights in a Court of Law and/or Equity, IT IS NEITHER LIKE SLAVERY NOR LIKE NAZI GERMANY.
Yes, I understand that hyperbole is a common argumentative tactic, but we have to remember that the Holocaust and American Slavery were so awful, that they have psychologically damaged the psyche of the descendants of the victims for (in the case of slavery), over a hundred years. Anyway, I realize that my words will have little effect on the scope of our political discourse, but I figured I'd try.