It’s a problematic article, with Frum trying to pull up the ladder he climbed and hoping no-one will notice. I don’t quite agree with haircut_hippie’s assessment, but the bit he’s referring to is close enough:
With immigration pressures bound to increase, it becomes more imperative than ever to restore the high value of national citizenship, not to denigrate or disparage others but because for many of your fellow citizens—perhaps less affluent, educated, and successful than you—the claim “I am a U.S. citizen” is the only claim they have to any resources or protection. Without immigration restrictions, there are no national borders. Without national borders, there are no nation-states. Without nation-states, there are no electorates. Without electorates, there is no democracy. If liberals insist that only fascists will enforce borders, then voters will hire fascists to do the job liberals refuse to do.
The point Frum deliberately misses is that valuing one’s U.S. citizenship (i.e. showing allegiance to the Constitution) is supposed to be at odds with electing authoritarian demagogues and fascists. If a member of the Know-Nothing 27% (or of Karen Stenner’s 33%, per Frum’s article) is insulted when that fact is pointed out to him after he votes for a right-wing populist, that’s not the fault of the bad ol’ educated and successful liberal who opposes fascism. One doesn’t need to be an advocate of totally open borders (a straw man 99% of the time it’s evoked) to understand this.