In addition to lack of enforcement at either the border or employers, under Bush's direction, the Treasury Dept issued regulations specifically endorsing Matricula Consulars for banking ID purposes. This was necessary for illegals to open bank accounts, send foreign wire transfers, and get mortgages/car loans.
In other words, Bush attempted to integrate illegals into our society so that it would be fait accompli by the time Congress got around to the issue. If you want to find the ONE person who is really responsible for the last 6 years, look no further.