~3.5% of the US population is Illigal immigrants. If they want to come here they have. In fact over the last five years more people have moved from US to Mexico than Mexico to the US.

It's not really an issue because our 11 million illegal immigrants don't just desired to move to Montana they do what other people do and arrive in waves. Only issue US has with internal migration has to do with Job openings which are fairly easily to predict if hard to deal with. EX: Boom towns from fracking.