What would be a good argument against this?

Both are somewhat true.Immigrants did take jobs no one else wanted early on in our countries development.Pay was not the only reason these jobs went unfilled.They were dangerous,performed under adverse conditions and sometimes required the worker to be away from his or her families for long periods.Plus there were more jobs than job seekers.There was a shortage of labor to do the dirty jobs then.Now things have changed.Manufacturing has fallen off and there are more job seekers than jobs.