The first is that it intrudes on the authority vested in the executive branch by the Constitution, which gives the president responsibility for executing the laws. Federal prosecutors normally deal with all sorts of controversial and politically sensitive cases while answering to their superiors, in a system that fosters a due sense of priorities. Special prosecutors (and special counsels and independent counsels — the distinctions aren’t crucial here), lacking the usual limits in latitude, are more likely to run out of control — as Kenneth Starr did, running a probe of Bill and Hillary Clinton that ate up six years and tens of millions of dollars but led to no charges against either.