>I admit I'm having trouble.

ca 1900 immigrants were citizens, yet still represented "free" labor, since the level of public entitlements at the time was essentially nil. Current illegal immigrants fill a similar niche, but that wouldn't remain true if they were suddenly granted citizenship, or if the U.S. were to suddenly adopt a policy of ulimited legal immigration from Mexico.

Consider the working poor (citizens) who receive welfare, live in subsidized housing, etc. Say there are 30 million such people. Their net per-capita contribution to the tax base, after you subtract out any entitlements they receive, is either negative, or if positive is substantially smaller than middle and upper-class citizens. I'm fine with that. But let's say that instead of 30 million there were all of a sudden 60 million, with a high unemployment ratio to boot. It would be a serious strain on the economy.

There are two main reasons we have "only" 10-20 million illegal immigrants (instead of many more):

1. That's all the job market will support, and since illegal immigrants are by definition not citizens (and thus not able to take advantage of the entitlements that otherwise be available to them) it's not feasible for them to remain (or come) here without a job.

2. It sucks being illegal and having to live as a fugitive 24/7.

Take away those two disincentives and I'm convinced the population of low-income low-education legal immigrants from Mexico would skyrocket far past the current population of illegals.