Democrats need more taxpayers. Same thing Reagan wanted for his tax reform.
And more working, taxpaying citizens is a bad thing because........?
The most obvious anti-immigrant talking point is that they push wages down. And that's probably true. We also already have an enormous oversupply of workers. Business would love to be able to hire these folks for peanuts without the risk of bad press.
You do realize that once they are legal, the leverage that employers use to pay them peanuts no longer exists, don't you?