Conservatives believe that immigrants are taking jobs, getting tax funded benefits, and are generally opposed to brown people doing something technically illegal.
In truth the majority take jobs conservatives don't really want, they do take tax funded benefits but also pay taxes in most cases (and they don't get the payouts for them most of the time), and the process for legally entering the country is so much more difficult than it should be that many resort to illegal immigration.
Oh, and going back to the benefits part, last I checked the average illegal immigrant household (on average) gets something like < $5,000 per year. Most of the time the benefits they are getting are things like SNAP or medicare because someone in the household is a legal resident and thus can qualify. When you apply for benefits they typically only care if the applicant is legal, but they count the whole household.
The whole thing is stupid, conservatives want to complain about pennies when we have dollar problems and at the end of the day immigrants prop up the economy. We honestly should be lowering barriers to legal entry.