IMO...I think they have made the assesment and have decided that the consequences of closing the borders are more severe than not. If the borders are closed, and millons of mexicans have to go home, the powers have concluded that the American economy will be severly hurt. Restruants will have to close, houses will not get built, yards will not get mowed, ditches will not be dug, roofs will not be installed, cars will not get washed, etc, etc.
I think that the Bush admin has decided that if they close the borders and the economy tanks, then the R's will lose across the board. If they don't, they may be able to overcome the voter fraud because of the laws that are popping up requiring proof of citizenship.
What they are finding out; however, that We, the People are getting involved and insisting that the borders be closed. It is an interesting problem. I want the borders closed...we can deal with the fallout when it happens.