Krugman uses 'racist' against those who are wanting tighter immigration enforcement.    There are lots of people, and I won't try to quantify, that are protectionist, nativist, xenophobic, legalistic, but not prejudiced specifically by race.     

Krugman stereotyping the target the issue as low skilled Latin labor, a specific subset of immigrants.    It's that subset that he says that brands conservatives to racism?   (Do we hear about European or Asian illegals - no).

It's Latin immigrant groups that most concerns those who want stricter immigration enforcement, IMO.     If that's the case there is a considerable amount of racism toward Latin cultures and it shows up as the 'negative only' stereotypes written by people on this site (I'm NOT saying ALL conservative writers on this site). 

Immigration policy is large and complex, not a simple declaration of law.  Congress, will fail to pass comprehensive reforms because it will require non-political expertise and administration.