Donald Trump is leading the field for the 2016 Republican presidential nomination by a wide margin. Of his policy positions, nothing has distinguished his campaign more than his ultra hard-line immigration proposals. These two facts are mostly undisputed, but so far commentators have focused on how both a Trump presidency and the imposition of such strict border controls are unfeasible. It’s a good way to avoid reckoning with their substance and consequences. When we’re talking about millions of lives at risk, however, it pays to be mentally prepared.

In order to “make America great again,” Trump wants first to better define it. Candidate Trump, if elected, plans to build a wall around the country. He also wants Mexico to pay for the construction costs. It’s a ridiculous, albeit somewhat popular, project, but the wall deflects attention from the rest of his dead-serious immigration plan.

Trump has accumulated a lot of support from the GOP nativist base in large part because they trust him to enforce immigration laws. The first sentence in his plan is “When politicians talk about ‘immigration reform’ they mean: amnesty, cheap labor and open borders,” and he’s right. Few national politicians and fewer business leaders are serious about deporting 11 million people. Undocumented immigrants are a necessary part of the national economy; American enforcement practices are designed to manage, not eliminate, violations of border laws.

A strain of American resentment can’t deal with this contradiction: If some people are allowed to violate the law, they must be doing so at the expense of lawful citizens. It’s not true; undocumented immigrants (not to mention over 50 million Latino Americans nativists are really talking about) contribute to the national economy, tax base, culture and social fabric in innumerable ways. But racism and xenophobia make inspiring campaign themes. “Enforce the law,” they say. But “enforce the law” would mean splitting the country in ways that are so horrific, they are difficult to contemplate.

First of all, there’s the scale. Deporting 11 million people would be a population transfer so large it only has a couple historical precedents, and one of them is Adolf Hitler's. To extract that many people from their communities would require a much larger and more determined effort that Immigration and Customs Enforcement (ICE) is capable of at present. To this end, Trump proposes tripling the number of ICE officers, specifically geared to increase deportations. In a country with volunteer border patrols and lasting unemployment, I don’t think President Trump would have a problem recruiting.

Sending an amped-up ICE on a mass-deportation mission wouldn’t just be an assault on undocumented people and their families, it would be an attack on American cities, where more than 90 percent of them live. For large municipalities, rigorously enforcing immigration law is unfeasible but also politically unpopular. So-called “sanctuary cities” have declared their ongoing intention to drag their feet when it comes to cooperating with the Feds. For example, law enforcement in many cities (including New York) selectively complies with ICE requests to hold people in custody on suspicion of being undocumented. ICE can’t do their job without local cooperation and the use of these legally questionable detention orders has decreased by more than 70 percent in the last four years.