On December 7, Donald Trump strode out in front of the assembled television cameras and did something unusual: The stream-of-consciousness candidate read a prepared statement.

It said: “I, Donald J. Trump, am calling for a total and complete shutdown of Muslims entering the United States until our country’s representatives can figure out what the hell is going on.”

Trump didn’t break a law that day; he broke a norm. Major politicians cansingle out particular religious groups for discriminatory treatment in the United States. They just … don’t.

They don’t do it even when they might find votes through bigotry. The restraint isn’t entirely altruistic. Norms are enforced through powerful, if informal, mechanisms in American politics — in this case, widespread condemnation from politicians of both parties, as well as (accurate) accusations of bigotry and racism.

But Trump isn’t bothered by the guardrails of American politics. As I’ve written before, his most salient characteristic is that he operates entirely without shame. He doesn’t care if he’s condemned, or called a bigot, or shown to be a liar. He cares about the polls, and he cares about winning, and he sees everything else as negotiable. In this, he truly is a dealmaker: If the cost of the presidency is being seen as a racist, he’ll pay it.

To understand the danger Trump poses, though, it’s important to recognize what happened next. Banning all Muslims from traveling to the United States, for any reason, went instantly from being a proposal so bigoted and outlandish that no one had even considered it to a proposal at the center of the American political debate. It was discussed. It was polled. It was normalized.

This is the lasting damage that Donald Trump has already done to our political system. Operating entirely without shame, he has normalized bigotry and hate. He has brought the racist, misogynistic, homophobic fringe into the mainstream. This is what makes this election historic. Trump’s hateful bigotry must be called out in the plainest terms and Hillary Clinton must be the one to do it. Josh Marshall nails this important truth on Talking Points Memo.

Donald Trump has not only brought haters into the mainstream, he has normalized hate for a much broader swathe of the population who were perhaps already disaffected but had their grievances and latent prejudices held in check by social norms. This isn’t some minor point or critique. It’s a fundamental part of what is at stake in this election, what makes it different from Obama v Romney when we had a critical election but still one that was mainly about different policy directions for the country. This election has become a battle to combat the moral and civic cancer Trump has injecting into the body politic. (I know that sounds like florid language but it is the only fitting and valid way to describe it.) Backing down would make Clinton appear weak, accomplish nothing of value and confuse what is actually at stake in the election.