I dont' think they are talking about the birth of illegal people. My take on their arguement is: if the US had not allowed 12 million abortions (you pick the number) over the course of the last thirty years, we would have that many more people amongst our ranks, and there wouldn't be any reason for the illegal immigrants to have come here in the first place.

             As for sources, I've heard a long string of right-wing-religious-wakkos making this claim, though I'd have to search to find one. I'll give it a quick look.