I 100% agree that we as a mature society, should be there and be prepared to tell them why they are wrong. But I think you are overlooking the possibility that they shrug and start killing. We should be prepared for that too.
If it comes to judging „what a truly tolerant person should do“ you make an absolute normative, hence moral claim.
This strikes me as rather inconsistent and therefore unconvincing.
On being 'confronted' with the question about legalizing genocide. Well, if there ARE people with such ideas, it's not like making the ideas illegal will stop such people from having them. So open discussion about it and democratic voting will have just 2 outcomes: nazis will be either reduced to nothing and their political base will dissolve, or they'll get to power and a genocide will happen. However, I believe that a society that will openly vote for genocide is beyond saving, whether you criminalize some ideas or not.
> What if somebody runs on that very platform issue for congress -- legalizing the murdering of jews?
There's nothing illegal about wanting to change the law to make something which currently illegal into something legal. All major changes in society the last century has happened by doing so. Think about cannabis-legalization if you need a slightly less morbid, but more recent example.
If I understand your point of view correctly (and I hope I don't), by your logic it should be illegal to argue legalization of anything which is currently criminalized. That would IMO be conflating legality with morality, a common error, especially among the conservatives.
Obviously anyone should be allowed to argue that there are reasons for why killing jews (or Canadians, or white people, whatever) should be legal. And we should, as a mature society, be there and be prepared to tell them why they are wrong.