I would vote for such a law. If someone expects government assistance, why can't the government expect them to be sober? Too many people in this country want a free ride. The middle class pays for everything. I can't go to my job under the influence of anything, can you?
My debate team had this as a topic this year. One of the most convincing arguments against it was that the cost to drug test everyone regularly would be prohibitive, compared to the cost of interventions if drug users are found. The actual proportion of drug users is pretty small and not worth the huge added expense.
I am generally against this sort of a law. First of all, I think that it is not really appropriate unless we are going to drug test everyone who gets any benefit from the government. For example, I get a child tax credit. Shouldn't the government drug test me to make sure that money is not going to someone who is a druggie?
Also, I really think it is just a political thing meant to try to demonize people on welfare. I doubt the law would really do much to help our economy or to help reduce the deficit.