I tend to think it's votes on the D side of the aisle and money on the R side. I don't think it's some grand conspiracy involving both parties getting together in some smokey room and coming up with some plan. Both sides have their reasons for wanting to keep the floodgates open.
None of it makes any sense at all for the GOP. Even if they could get conservatives to blindly vote for them, it still won't overcome the illegal voters coming across the border.