Border control is only one facet of the problem.
Let's say Bush were to actually decide that the US is a country of laws and that he was going to enforce them.
Let's say that Bush systematically ran the illegals out of the US. I personally think this could be done inside of a year, maybe two years.
There would be an immediate impact on the (profit) prices of many everyday things. Houses and agricultural products being two prominent areas that would be quickly affected.
This immediate increase in prices would not be offset by a reduction in cost to the citizens from State and local governments nor hospitals nor any other group burdened by illegals. When is the last time you ever actually saw government get smaller year over year?? Everyday citizens would see an everyday increase in the cost of many things.
Bush won't take the heat for that change. He thinks its better to leave the weight of these illegals on the backs of the citizens of this country than it is to enforce the laws he swore to uphold.
I personally think he is as out of touch as his father was, only in different ways. He is going to find his legacy to be quite different from what he thinks it will be. And in the process he is going to hand the presidency to the democrats. A third party that promises to do something about the problem could easily deflect enough votes from the Republicans to hand the election to the Dems.
I could be wrong, there could be some really imaginitive reason that escapes me. But this simple answer fits well.
I think you've nailed down the election for '08. What a pity.