The natural state of all government is to be corrupted by moneyed interests.
It would be pretty easy to 'prove' this by simply pointing to every government throughout history and daring any skeptics to find one that doesn't fit the pattern. They would, of course, fail, but absence of evidence is not evidence of absence.
More difficult, yet ultimately more satisfying, is to lay out a logical foundation showing how admitted human nature inevitably leads to a large, unwieldy, inefficient, and thoroughly corrupt government. The starting point is as J.E.E.Dahlberg (Lord Acton) once warned us: 'Power tends to corrupt; absolute power corrupts absolutely', and the essence of government is power — over people, over the economy, over corporations. Aided and abetted by a population insufficiently wary of government's tendency to grow, government will grow incrementally at first, then by leaps and bounds until powers never intended for government to have will now seem ordinary and everyday functions of government. Normalized by long-use, functions that once were solely the province of non-governmental entities appear so natural for a central government to handle that people forget the times when government didn't do such things.
This has been the path followed by all governments — ours is not an exception — as they grow from cottage industries to leviathans. The reason is intuitively obvious. Friedrich Hayek offers an alternative view to that of Acton: government itself does not corrupt, but the power inherent in government tends to attract the corruptible. That is: bureaucrats start out corrupted and gravitate toward an environment where their natural corruption is both tolerated and nourished. Someone, say that this is not so. Experience has shown us that it is the most true thing one can say about government, not just our government; all governments.
It's not even certain that a Libertarian administration — populated by people who are ideologically committed to minuscule government — would not fall into this same trap.
But it might be worth a try.