Frum argues, rightly, that several of the concerns that Trump advanced ought to be part of a rational immigration policy debate in the United States. Low-skill immigration poses real fiscal costs by adding residents who consume more in services than their taxes pay for. Assimilation is more problematic in some immigrant communities than others. Unlike other policy debates, the immigration debate often is oddly focused on the interests of non-citizens.
Frum is right that these points should be discussed as part of a debate over what immigration policy to have. The problem is that Trump's story about why immigration should be restricted goes well beyond these arguments, into territory that is shocking and dangerous.