Now, most Sheriffs are political minded folks and would require the backing of some of the officials in the County and preferably some in most of the towns and cities.
In Nevada they are up for a four year term, but I think in four years they could prove that this idea will work, especially to the taxpayers.
The reason I choose a lesser populated County is that way you have less people to convince and less people against you, also there is a smaller amount of taxpayers who currently are paying for these illegals.